Arsenal have turned down Manchester United's opening offer for Samir Nasri and told the champions they will not sell the Frenchman unless they receive a bid of at least £25m. United must now decide whether they are willing to meet the asking price and put together an improved financial package after it emerged on Wednesday they had placed an offer of just under £20m two weeks ago.

The clubs had agreed at the time not to publicise the bid but it is now clear that United, having already spent £50m on Phil Jones, David de Gea and Ashley Young, have made the first decisive move towards bringing Nasri to Old Trafford and reinforcing their position as the major force in the summer transfer market so far.

At the same time, they are threatening to make it an even more traumatic summer for Arsène Wenger, who is already faced by the near-certainty of Cesc Fábregas's departure to Barcelona and now confronted by the distinct possibility of losing another of his key players. Nasri is due to return to Arsenal for pre-season training on Thursday and the expectation is that the 24-year-old, in the final year of his contract, will inform the club he wants to leave. Arsenal are willing to award him a vastly improved salary but they will still find it difficult to compete with the money on offer from Old Trafford.

The news of United's bid may trigger counter-offers from Manchester City, Internazionale and Chelsea, who have monitored Nasri's potential availability and are weighing up whether to turn their interest into something more concrete.

City distanced themselves from reports last week they had made a £20m bid, saying there had been no talks between themselves and Arsenal and that, at that stage, none were planned. They are, however, reconsidering their options after the failed move for Alexis Sánchez of Udinese. Should they decide to compete against United, City would also be confident they have the financial power to offer Nasri the best deal. The difficulty for Arsenal is that Nasri will be available as a free transfer in a year's time and, unless he signs a new contract, his value will start to depreciate sharply. Wenger, therefore, must decide whether it would be better for the club to cash in on the player and put the money towards rebuilding.