There was a time not very long ago when the former Leeds United idol Alan Smith was being courted about a return to the club’s coaching staff. Smith, working in youth football in Florida, was identified as someone with the background, pedigree and personality to inspire the next generation of young Leeds prospects.

‘We couldn’t make it happen,’ a source close to the negotiations tells Daily Mail Sport. ‘I am sure there were several reasons but Alan has never forgotten how his family were abused after he moved to Manchester United back in the day. We believed that was still in his mind.’

Leeds United and Manchester United. Colourful, bitter and deep. There are few football rivalries like it.

‘When I was in the academy at Leeds you knew all about it,’ recalls former Leeds goalkeeper Paul Robinson.

‘It didn’t matter how young you were. It was there. Back then it was an option for us to be ball boys at home games. Some lads couldn’t be bothered. Sitting there in the cold and wet.

‘For Man United games, it was suddenly oversubscribed and we all wanted the spot in front of the away fans. You just wanted to feel it and be in the middle of it.’

Leeds will host their rivals from across the Pennines at 2pm on Sunday. It feels like another big test for Michael Carrick and his team but inside Leeds’ fabulous old ground, it will be no more frenzied than normal simply because that is just not possible. Even hate has a ceiling.

What has always been less clear is why the rivalry goes so deep. The two clubs do not share a city or even a county. There is not a single seismic event or game around which such mutual enmity can be built.

Some talk about the day Eric Cantona left Leeds for Old Trafford in 1992 as a pivotal moment and indeed the moment United captain Roy Keane first clashed with Alf Inge Haaland during a game five years later.

But the truth is the loathing and suspicion goes back way further than that. Even former United manager Sir Alex Ferguson was surprised by it when he arrived from Scotland in the 1980s.

Hit on the head by a cup of Bovril after being called a ‘Manc b******’ on an early scouting mission to watch Leeds play Crystal Palace, Ferguson returned to his club wondering just where all that came from. ‘It’s from the 1960s,’ said his kitman Norman Davies.

Talking to those on both sides of the divide, there seems truth in that.

Some talk romantically about the tone set by the War of the Roses – Leeds do wear Yorkshire white and United Lancashire red – but it seems fanciful that a dislike so visceral can really be tracked back to the mid-1400s.

More significant may have been Johnny Giles’ switch from United to Leeds in 1963. Giles became synonymous with Leeds’ aggressive style of play as they twice won the English title, in the late 1960s and early 1970s.

Former United midfielder and European Cup winner Pat Crerand is blunt about all that.

‘Don Revie’s Leeds team never got the credit they deserved, but maybe that’s because of the way that they behaved,’ Crerand has said.

‘People had grudging respect for the success we had at United under Sir Matt Busby because of the way we played.

‘But there was no respect for Leeds and that partly explains the rivalry.’

The reverence duly served to United teams that played in the Busby era and indeed under Ferguson stands in contrast to the enduring image of the ‘dirty Leeds’ of Revie’s days.

It is probably unfair but public perception and what it means was a theme touched on this week by perhaps Leeds’ most famous supporter.

Gary Edwards is an author and painter and decorator who has not missed a single Leeds game – bar the Covid era – since 1968. Amusingly, Edwards refuses to paint anything red and has been known to paint over the colour for no payment.

‘I only do one coat for free,’ Edwards says with a laugh. ‘But yeah, the rivalry is deep and from our point of view there is that feeling Man United have always felt they have a god-given right to anything.

‘They fancy themselves, don’t they? Maybe they do try and look down on us a bit. They feel that everyone in football should bend over backwards for them.

‘It’s not jealousy from us, as some people say it is. When I started going to matches, we used to beat them regularly.

‘But here in Leeds we just feel we don’t lie down for anyone, especially not them.’

In Manchester, those who have crossed the divide still talk about it. Lee Sharpe – who is actually from the Midlands – left Old Trafford for Yorkshire in 1996 and still gets called ‘scum’ by his mates.

The late Gordon McQueen – a Scot – went the other way in 1978 and found himself verbally abused so incessantly at a pre-season game 30 years later that he decided to go home.

Ferguson, meanwhile, could never quite get his head round it. Sources this week revealed to Daily Mail Sport that one of the reasons the two clubs have always managed to exist cordially at boardroom level is because of the friendship between Ferguson and his fellow Scot Eddie Gray, a Leeds club ambassador.

‘Eddie and Sir Alex are part of the glue between the clubs,’ says a source from the modern Leeds era. ‘They have a really good and funny relationship going back to their days in the game.

‘They will chip away at each other but in the boardroom the sight of those two sitting at a table winding each other up keeps the mood exactly as it should be. It takes the edge off everything. They are desperate to beat us, mind.’

Ferguson is these days known to light-heartedly mock the quality of the wine served in the Elland Road boardroom but was not always so generous. His two autobiographies are regularly pillaged for anecdotes – less so a diary of the 1994-95 season that contains this startling extract after a defeat in Yorkshire.

‘You can feel the hatred, the animosity,’ writes Ferguson. ‘ I said to the players afterwards, “Now you know what it’s like to lose here”.

‘The Leeds supporters were awful. I almost want them to be relegated because of their fans. I don’t think they deserve to be in the Premier League. Such hatred really is incredible.

‘A TV camera caught a woman screaming abuse at me. What chance have you got? You could understand the animosity better if it was Liverpool and United because they are just down the road from one another.

‘You can’t say the same for Leeds. You are expecting a pitch invasion at any moment.’

In Yorkshire, there is an acceptance that things have sometimes crossed the line. Even when the teams met at Elland Road during the Covid lockdown, Leeds supporters tried to gather outside to ‘greet’ the United coach.

‘It’s not just one way, though,’ adds our Leeds source. ‘I have been in the directors' box at Old Trafford and received dog’s abuse. Never in my life have I experienced anything like that.’

Sunday’s meeting should be tamer on the field than some have been. In 1965, for example, an FA Cup semi-final featured a punch-up between Jack Charlton and Denis Law. Leeds won the replay 1-0 but lost the league title on goal average to their rivals. Four years later, in January 1969, crowd trouble at Elland Road during a match between the sides left 50 in hospital.

Leeds’ modern travails mean that not often in recent years have the teams competed at the same level – they met just twice between 2004 and 2020 for example – but tensions have endured.

Former Leeds midfielder Kalvin Phillips, for example, refused to countenance a switch to United when they expressed interest in 2022. He moved to Manchester City instead.

Meanwhile, two years earlier Marcus Rashford was compelled to remove a social media post welcoming Leeds back to the Premier League following a backlash from fans of his own club.

‘If you play for either club you have no choice but to understand it,’ adds goalkeeper Robinson.

‘When Leeds were two divisions down, Man United fans were still singing about how much they hated them.

‘It’s about keeping up tradition isn’t it? Modern-day Millwall fans think they have a tradition to uphold and it’s the same at Leeds, even if the actual origins have got a bit lost.

‘It’s always Leeds’ biggest game and that filters through to the dressing room. It’s like nothing I’ve ever known.’

Guest of honour for Sunday's game will be former Leeds and United midfielder Gordon Strachan. The 69-year-old was captain of Howard Wilkinson’s Leeds team that pipped United to the title in 1992. Cantona left the following autumn and a period of Old Trafford dominance followed.

‘I still can’t look at Howard the same way after that,’ admits Edwards. ‘It didn’t start the rivalry, though. No, it was long before that. I went to my first game in 1968 and it was there then.

‘I actually feel a bit sorry for Man United right now and that’s a strange feeling.

‘I went to a game at Old Trafford last season and they were wearing half-and-half scarves.

‘I’d go there thinking I may get lynched. I’d rather that than wear one of those things. There will be none on sale on Sunday. That’s not what we do in Leeds.’