Arsenal's Aaron Ramsey put a huge dent in Manchester United's title charge and boosted Chelsea's challenge as the Wales midfielder secured a 1-0 victory over the leaders at the Emirates Stadium on Sunday.
United arrived in north London seeking the victory that would restore their six-point advantage over second-placed Chelsea with three games remaining and put them within touching distance of a record 19th league title.
Instead they departed with the title race firmly back in the balance after Ramsey's strike - his first for the Gunners since suffering a broken leg in February 2010 - ensured that Carlo Ancelotti's Chelsea will visit Old Trafford on May 8 knowing victory will take them to the top of the table on goal difference.
Victory also provided a lift to Arsene Wenger's Arsenal, who had fallen nine points off the pace after a miserable run.
While the chances of them overhauling United and Chelsea are slim, Wenger will insist Arsenal compete until the maths means they are destined to finish also-rans.
United believed they should have had an 88th minute penalty when Michael Owen appeared to be tripped by Gael Clichy.
However, Arsenal were denied a similar claim in the first half when United defender Nemanja Vidic appeared to handle and overall Sir Alex ferguson's side could have few complaints after a strangely subdued display.
Arsenal were deprived of their skipper Cesc Fabregas after their captain was kicked on the thigh in training on Saturday.
But the home side quickly shrugged off the loss of the Spain international, immediately taking the game to United and carving out a succession of early chances that should have seen them establish an early advantage.
They were helped on their way by Vidic whose poor clearance in the third minute fell to Jack Wilshere on the edge of the area and the young England midfielder should have done better than pull his shot wide.
Having been so impressive in winning at Schalke 04 last week, United looked disjointed as they attempted to re-establish their advantage at the head of the table.
Ferguson had selected a full strength line-up, even though his side face Schalke in the second leg of their Champions League semi-final at Old Trafford in midweek.
But they had to wait until the 21st minute before they made their first inroads against the Arsenal defence, with Gunners full back Bacary Sagna forced to react quickly to prevent Fabio Da Silva's cross reaching Wayne Rooney.
That proved to be a fleeting moment of United pressure and, having regained control, Arsenal were convinced they should have been awarded a penalty when Vidic's hand took Walcott's cross away from the forehead of Robin van Persie.
Van Persie was just six yards out and would almost certainly had scored had Vidic not intervened but referee Chris Foy and his assistant failed to see the incident.
Arsenal have suffered this season because of their failure to build on promising starts and Rooney's free kick three minutes after the break, excellently saved by Wojciech Szczesny, served as a reminder that could again be the case.
But Wenger's side were playing like they had a point to prove and they deservedly forced the breakthrough just before the hour.
Van Persie worked his way down the right hand flank and with United backtracking, pulled the ball back for Ramsey who was lurking on the edge of the box.
Michael Carrick raced out to close the youngster down but was too late to prevent Ramsey shot arrowing into the corner of Edwin van der Sar's goal.
The Gunners held on without too many alarms, although Szczesny had to save from Nani before United's appeal for a penalty was rejected following Clichy's challenge on Owen.