The News of the World says an ugly punch-up erupted on the pitch with Chelsea groundstaff minutes after the Blues had won 2-1 to pull level with United at the top of the Premier League.
United players were warming down after this top-of-the-table clash. Patrice Evra, Paul Scholes, Gary Neville, Gerard Pique, Ji-Sung Park and John O'Shea were jogging on the pitch when an argument with groundstaff boiled over.
One eyewitness said: "It was amazing. One minute the United players were running up and down, the next minute it was a free-for-all. This was not handbags - this was a real dust-up involving players and groundstaff.
"First of all words were exchanged but then the punches started flying."
Unofficially, the Blues' version of events is that the club's senior groundsman asked the United players to use another part of the Stamford Bridge pitch for their warm-down.
One senior United player is then said to have given the groundsman short shrift - and that is when events turned nasty and a confrontation quickly ensued.
Chelsea are adamant that Evra threw the first punch, striking a member of their groundstaff in the face before being pulled back and then running off. United say that it was Evra who was punched.