LONDON, May 25 (Reuters) - Manchester United ( MANU ) saved their best performance of a mediocre season to the very end as they upset Manchester City 2-1 in the FA Cup final on Saturday to deprive their rivals of the double.

With speculation swirling about the future of manager Erik Ten Hag, United ripped up the form book as first-half goals from teenagers Alejandro Garnacho and Kobbie Mainoo proved enough for them to lift the trophy for a 13th time.

They had to endure a second-half siege, however, as City swarmed forward and a late goal by Jeremy Doku set up a nerve-shredding finale before United could celebrate at Wembley.

Premier League champions City, bidding to become the first club to win the double in successive seasons, had been on a 35-match unbeaten run in all competitions in open play.

But they were well below their best in the first half, gifting Garnacho a 30th-minute opener before Mainoo made it 2-0.

City woke up in the second half and Erling Haaland hit the crossbar, but they had to wait until the 87th minute for substitute Doku to halve the deficit.

United hung on, however, to seal the best result of Ten Hag's two-season reign and qualification for next season's Europa League.

"Incredible. Nobody believed in us. But we're a team, all together. We fought, the game of our lives," Garnacho, who became the first Argentine to score in the Cup final since Ricky Villa for Tottenham Hotspur in 1981, said.

The build-up had been dominated by talk of the chasm between the two Manchester clubs with City having won an unprecedented fourth successive English title and United suffering their worst Premier League campaign.

FUTURE DOUBT

Ten Hag's future was also clouded in uncertainty with reports that the showpiece would be his last game in charge, whatever the outcome.

Even the most optimistic United fans would have walked down Wembley Way in the sunshine fearing a drubbing, but their team turned logic on its head.

Garnacho, one of the few positives in United's season, had the game's first shot on target and his movement was a constant threat to a City defence featuring two changes with John Stones and Nathan Ake handed starts.

City looked ruffled by United's energy and offered a rare gift after 30 minutes.

A long ball forward should have been easily dealt with by Josko Gvardiol but his headed back pass went over advancing keeper Stefan Ortega and Garnacho ran through to tap the ball into an empty net.

Marcus Rashford had a goal ruled out minutes later with Garnacho adjudged offside in the build-up but it was not long before United were in dreamland.

This time there was no help required as a brilliant move saw Garnacho play the ball in to Bruno Fernandes whose no-look pass released Mainoo to slot his shot past Ortega.

City moved up several gears in the second half with substitute Doku causing problems and it was from his pass that Haaland smashed a shot against the crossbar.

United keeper Andre Onana had to make a superb save to keep out a Kyle Walker effort and Julian Alvarez, who came on for Kevin De Bruyne, fired a shot over as City began to exert real pressure.

Alvarez side-footed wide from in front of goal but United's rearguard began to wobble and City finally broke United's resistance when Doku cutting in to drive a shot past Onana but they had left it too late.