Sir Alex Ferguson's side were making their way to Euston for their return journey to the north-west when a bottle was hurled at the windscreen.

No-one was injured, although the windscreen was cracked.

Nevertheless, the team continued on their short trip across London without further incident.

United officials were eager to play down the incident afterwards.

"Yes. There was an incident but the matter is now in the hands of the police," said a club spokesman.

Last time United visited the stadium Patrice Evra became involved in a heated post-match exchange with Chelsea groundsman Sam Bethell which is still the subject of an FA investigation while Rio Ferdinand was forced to apologise to a female steward after kicking a ball at her in anger.

Sunday's encounter was not much less contentious, with United picking up seven yellow cards from referee Mike Riley which will now see them fined by the FA.

A Chelsea statement read: "We are aware of reports of an altercation with the Manchester United bus as it left the Stamford Bridge area.

"It appears to be an isolated incident but we have launched a full investigation in conjunction with local authorities.

"We do not condone violence in any shape or form. If, as a result of our investigations, we find that any Chelsea supporters were involved we will take appropriate action against those individuals."