After tipping Manchester United to win the title on the eve of his return to Old Trafford, Steve McClaren then dealt a massive blow to his old club's championship chances.

McClaren, who left United in the summer to become Middlesbrough boss, claimed the Red Devils had the nerve to win a record-breaking fourth successive championship.

However, that nerve failed United when they needed it most as Middlesbrough recorded only their second league win at Old Trafford since 1930.

In January, Middlesbrough had dumped United out of the FA Cup to end their dream of another Treble.

This eighth league defeat, courtesy of Alen Boksic's ninth-minute winner, could have far more serious repercussions and the loss leaves United just one point clear of second-placed Arsenal having played two games more.

United hold just a two-point advantage over Liverpool and the Merseysiders could depose them as Premiership leaders if they beat Chelsea at Anfield tomorrow.

Worryingly United must play Leeds, Chelsea and Arsenal - at a date yet to be arranged following the Gunners' FA Cup success this morning - in their final six games and this was a game they could ill afford to lose at this critical stage of the campaign.

It had all seemed so different when United kicked off in front of their fans who were expecting to see their side chalk up their 15th league win in their last 17 games.

Ole Gunnar Solskjaer and Roy Keane were missing through injury, but United had welcomed back £47million worth of talent in the form of Juan Sebastian Veron and Ruud van Nistelrooy, who picked up his player of the month award for February before kick-off.

McClaren made two changes as Ugo Ehiogu returned from suspension and Noel Whelan from a virus.

Just like at the Riverside Stadium, United were undone by some poor play only this time at the start of the match rather than at the end.

Fabien Barthez played a short ball to Veron just outside the United box and he was robbed by Benito Carbone, who laid the ball across for Boksic to steer the ball home.

Remarkably, it was the fifth time in their last six visits that Boro had scored first at Old Trafford.

However they had only managed to go on and win on one of those occasions and that was the challenge now facing the Teessiders.

They threatened a second and the impressive Carbone played a delightful ball to Boksic down the left channel only for the Croatian to chip a weak shot into the arms of Barthez.

United managed their first real attempt at the Boro goal and Diego Forlan shot well wide - to the amusement of the travelling fans, who will never forgive him for snubbing them in favour of the champions.

Finally the Red Devils began to look more like their old selves and van Nistelrooy turned and fired narrowly before Ryan Giggs should have scored when he shot wide from Beckham's threaded through ball.

United's discipline began to slip as a couple of marginal decisions went against them.

Beckham was booked for the second successive week for dissent before Sir Alex Ferguson blew his top with the assistant referee for wrongly flagging the England skipper offside.

Veron joined Beckham in referee Steve Bennett's notebook for a foul on substitute Luke Wilkshire, who had replaced the injured Jonathan Greening, and United were in danger of being reduced to 10 men.

Boro sensed their disarray and Barthez denied them another goal when he made a smart save at his near post to deny Boksic before the Croatian striker steered Carbone's pass wide.

Referee Steve Bennett tumbled even further in Ferguson's estimation when he harshly booked Silvestre for handling Carbone's cross and then van Nistelrooy for scything down Whelan, although that caution was deserved.

Bennett continued to flourish his yellow card and Franck Queudrue was cautioned for a foul on van Nistelrooy.

Then on 58 minutes, Ferguson switched to plan B when he replaced the ineffectual Veron with Paul Scholes.

Still United pressed forward and still Middlesbrough stood firm with Ehiogu and Gareth Southgate outstanding for the FA Cup semi-finalists.

Mark Schwarzer had so far done relatively little in the Boro goal, but he proved his mettle when he made a fine save to block a van Nistelrooy shot.

By now Ferguson had left his usual position on the United bench and stood sentinel-like on the edge of his technical area, sensing the initiative in the title race was slipping away from his side.

By the time the final whistle blew, he knew it had.

Teams:

Man Utd: Barthez, Gary Neville, Johnsen, Blanc, Silvestre, Beckham, Butt, Veron (Scholes 58), Giggs, van Nistelrooy, Forlan (Fortune 82).

Subs Not Used: Carroll, Phil Neville, O'Shea.

Booked: Beckham, Veron, Silvestre, van Nistelrooy.

Middlesbrough: Schwarzer, Stockdale, Ehiogu, Southgate, Queudrue, Greening (Wilkshire 21), Ince, Mustoe, Whelan, Boksic (Windass 79), Carbone.

Subs Not Used: Crossley, Nemeth, Gavin.

Booked: Queudrue, Ince.

Goals: Boksic 9.

Att: 67,683

Ref: S Bennett (Kent).