Thomas Vermaelen
Thomas Vermaelen was desperate to leave ArsenalReuters

Arsenal have reportedly agreed to sell defender Thomas Vermaelen to Barcelona for a transfer fee of £15 million.

The 28-year-old, who has been keen to leave the Emirates after spending most of last season on the bench, is soon expected to fly to Spain to undergo a medical and agree personal terms, according to the BBC.

Arsenal manager Arsene Wenger had confirmed that Manchester United, who are in the hunt for a centre-back following the departures of Rio Ferdinand (Queens Park Rangers) and Nemanja Vidic (Inter Milan), had also made a bid for Vermaelen.

But the Gunners boss decided to sell the Belgian to Barcelona, insisting he was not comfortable selling the player to a direct rival.

Vermaelen, who moved to Arsenal from Ajax for £10m in 2009, fell down the pecking order last season, with Wenger preferring Per Mertesacker and Laurent Koscielny at the heat of the defence.

Meanwhile, Wenger admits that the Gunners need to be tight at the back against bigger teams in the Premier League this season.

Last season, the Gunners had suffered crushing defeats in the away games against Manchester City (6-3), Liverpool (5-1) and Chelsea (6-0), with the last two effectively ending their title charge.

And Wenger insists that they will not repeat the same mistake in the forthcoming season.

"I was concerned [by Arsenal's record against the top four]," Wenger told the club's official website. "I was concerned and shocked by the number of goals that we conceded."

"I think it was down to confidence. Overall we were quite sound defensively but we have a good opportunity to improve that."

Arsenal will play Turkish side Besiktas in the play-offs of the Champions League.