Louis van Gaal is keen to strengthen his defence following the departures of Rio Ferdinand, Nemanja Vidic and Patrice Evra and Wenger has now confirmed that the Old Trafford club have submitted an offer for the Belgium international.
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Wenger told reporters: "Manchester United are part of the group (of bidders). He could leave.
"We have offers from abroad and ideally I would rather sell him to a club abroad. We're in a position where it's difficult to stand in his way."
Barcelona are keen to bolster their back-line ahead of their bid to reclaim the La Liga title from Atletico Madrid and have identified Vermaelen - who made 21 appearances for the north Londoners last season - as a target ahead of the new season.
Meanwhile, Wenger confirmed that Arsenal have "more money to buy players now", having already acquired Alexis Sanchez, Mathieu Debuchy, David Ospina and Calum Chambers, though he hinted that he could resist the urge to strengthen further.
"We have three and a half weeks until the transfer market closes - we could still strengthen, but we have a big squad," he said.