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This all-League One tie turned out to be one of the most one-sided of the day, with Posh boss Robbie Savage watching on in horror as his side were pulled apart.
Reading went ahead after 26 minutes as slick passing led to Andy Rinomhota playing in Daniel Kyerewaa, who found the bottom corner via a deflection.
The Royals doubled their lead in the 34th minute when a brilliant cross from left-back Haydon Roberts was met by a diving header from top scorer Jack Marriott.
It was effectively game over in first-half stoppage time as Savage deepened his father’s misery when he took possession after the ball ran off Marriott, with his deflected effort finding the top corner.
Jamie Reid added a fourth for the visitors in the 72nd minute when Paudie O’Connor nodded Savage’s free-kick into his path for him to lift over Peterborough goalkeeper Alex Bass.
Reading weren’t finished yet as they struck yet again four minutes later when Liam Fraser’s pass put Reid clean through to score his second.
Robbie Savage’s Peterborough United side have endured a difficult recent run, failing to win any of their last five matches in all competitions.
Their 5-0 FA Cup defeat to Reading followed a 2-0 home loss against Luton Town and a 2-0 Carabao Cup defeat at Bradford City. Before that, Posh drew 1-1 away at Bromley and suffered a 4-1 defeat at Huddersfield Town.
That means Peterborough have lost four and drawn one of their last five, conceding 14 goals and scoring just twice.
Their most recent victory came on October 10, when they beat Leicester City 3-1 away in League One, Savage's first game in charge after leaving Forest Green Rovers.