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

Goals by teenagers Alejandro Garnacho and Kobbie Mainoo put United in the driving seat by halftime at Wembley and they survived a City siege after the break to win the trophy for the 13th time.

Champions City, bidding to become the first club to win the double in successive seasons, were well below their best in the first half and gifted United the lead in the 30th minute when Garnacho tapped home after a defensive mix-up.

United doubled their lead before the break as Mainoo finished calmly after a superb team move.

City woke up in the second half and Erling Haaland hit the crossbar, but they had to wait until the 87th minute for substitute Jeremy Doku to score and set up a tense finale.

United hung on, however, to ensure they will qualify for next season's Europa League.