LONDON, May 25 (Reuters) - Manchester City manager Pep Guardiola rarely puts a foot wrong but he took the blame as his side were deprived of a second successive English double by a shock 2-1 loss to Manchester United ( MANU ) in the FA Cup final on Saturday.

A week after wrapping up an unprecedented fourth Premier League title in a row, City were massive favourites to saunter past rivals United in the Wembley sunshine.

But their 35-match unbeaten run in all competitions in open play ended as first-half goals by teenagers Alejandro Garnacho and Kobbie Mainoo proved enough for United to end their disappointing season on a huge high.

"Congratulations to Manchester United ( MANU ) for winning the FA Cup. I think my game plan was not good," Guardiola, who has won 15 major trophies for City since taking over in 2016, told reporters.

"The second half was much, much better. We were more intense, in part because we were 2-0 down and had nothing to lose. It was a tight game.

"In general, it was a good performance considering the final, but sometimes this happens. We don't play against United, we play for ourselves."

Guardiola elected to change his central defensive pairing with John Stones returning alongside Nathan Ake but his side looked unusually vulnerable as United's plan to attack quickly on the break worked a treat.

City did find their usual tempo in the second half when Jeremy Doku came on and laid siege to United's goal but Doku's goal proved too late to salvage their hopes of becoming the first club to win the double in successive seasons.

They did have the UEFA Super Cup and FIFA Club World Cup titles to add to their collection this season, however, so while defeat will sting, especially to United, it will only take a little of the gloss off their campaign.

"Disappointed today, it's normal teams can lose finals but this season has been extraordinary fighting for all the trophies in a good way," Guardiola said.

"We will rest and come back next season."

Guardiola also had words of encouragement for United manager Erik ten Hag who could still face the sack despite leading his team to victory over City, who finished 31 points ahead of them in the league table.

"They have to take a decision. He's a lovely person and an extraordinary manager," Guardiola said. "Winning the FA Cup is important for them like last season it was for us."