Just as it seemed that Watford had done enough to earn a draw that would have allowed Manchester United to stretch their lead at the top to eight points, the Dutch striker drifted into the box to meet Andriy Shevchenko's cross and head the ball past United's on-loan goalkeeper Ben Foster.

Afterwards, Mourinho admitted that the title race would have been effectively over if Kalou had not taken his chance to ensure that the champions matched United's win over Blackburn earlier in the day.

"Mathematically we could still win because there are still 21 points in the game, but if you go to eight behind it is much more difficult," Mourinho said. "It was a crucial goal and a crucial victory."

Six points clear with seven matches to play, United remain firmly in the driving seat. But the last-gasp nature of Chelsea's win will only strengthen the champions' belief that they can close the gap.

United had to come from behind to beat Blackburn 4-1, but once they hit their stride they were irresistible.

A Matt Derbyshire strike had given Blackburn a half-time lead. But after a superb solo equaliser from Paul Scholes just after the hour, goals from Michael Carrick, Park Ji-Sung and substitute Ole Gunnar Solskjaer ensured an ultimately comfortable win.

Ferguson claimed his side had shown that they deserve to reclaim the title.

"You would expect a championship team to show that kind of nerve, determination, drive and resolve," he said. "We had all that. Everything you are looking for in a football team was there."

Ferguson was left with a potentially major headache however. Serbian defender Nemanja Vidic had to be stretchered off with a shoulder injury that will keep him out of action for up to five weeks, potentially upsetting the defensive solidity on which United's challenge for league, cup and Champions League silverware has been based.

Elsewhere on Saturday, a classic hattrick by Peter Crouch helped Liverpool crush Arsenal 4-1 at Anfield and leapfrog their London rivals into third place in the table.