Arsenal and Manchester United have agreed a swap that sends Alexis Sanchez to Old Trafford and Henrikh Mkhitaryan in the opposite direction.

Sanchez says he is thrilled to join "the biggest club in the world".

The Chile international sealed his switch to Old Trafford in a massive swap deal that sees Henrikh Mkhitaryan move to his former club Arsenal.

Sanchez's move ends months of speculation as his contract at Arsenal wound down well into its final year and comes after Premier League leader Manchester City had heavily pursued his signature since the previous transfer window.

After arriving at United, the 29 year-old was thankful to the Gunners, with whom he spent three-and-a-half years having signed from Barcelona, but felt the chance to play for the 13-time Premier League winners was simply too good to turn down.

"I am thrilled to be joining the biggest club in the world," the forward told United's website.

"I have spent three-and-a-half wonderful years at Arsenal and I bring with me very positive memories of that great club and its fans. 

"The chance to play in this historic stadium and to work with Jose Mourinho was something I could not turn down. 

"I am very proud to be the first Chilean player ever to play for United's first team and I hope I can show our fans all around the world why the club wanted to bring me here."

United boss Jose Mourinho, meanwhile, was delighted with his latest star acquisition and also wished Mkhitaryan well.

"Alexis is one of the best attacking players in the world and he will complete our very young and talented group of attacking players," Mourinho said.

"He will bring his ambition, drive and personality, qualities that make a Manchester United player and a player that makes the team stronger and the supporters proud of their club dimension and prestige.

"I would like to wish Henrikh all the success and happiness that I am sure he is going to get. He is a player that we will not forget, especially for his contribution to our Europa League victory."

Mkhitaryan expressed his satisfaction at completing the switch to north London, where he has signed a "long-term contract".

"I'm very happy that we could finish this deal and I'm very happy to be here," Mkhitaryan told Arsenal's official website following Monday's confirmation of the blockbuster switch.

"It's a dream come true because I've always dreamed of playing for Arsenal. Now that I'm here, I'll do my best for this club to create history.

"I always loved the way that Arsenal played and the way that Arsene Wenger has managed the team. There have been a lot of young players, he's managed them and they've done very well.

"Of course I was in love with the game of Thierry Henry. I loved the way he played - and maybe that was the reason to support Arsenal."

Arsenal manager Arsene Wenger is delighted to have captured Mkhitaryan and hailed the player's range of qualities.

"Henrikh is a very complete player," said Wenger.

"He creates chances, he defends well, he can absorb distances and he's very committed as well. I must say he's a player who has all the attributes."