Wayne Rooney has thrown Manchester United's season into a state of turmoil after informing the club he has no plans to sign another contract and intends to find new employers. Rooney's decision is based on serious differences with Sir Alex Ferguson, the Guardian understands, and will be a devastating blow to the supporters who have come to regard him as a talismanic figure in this troubled era under Malcolm Glazer's ownership.

United may have no option now but to sell the England striker, possibly in the January transfer window, rather than risk his transfer valuation dramatically lowering now that he is only 20 months away from becoming a free agent.

Rooney has always said he has no desire to play abroad and would like to remain in Manchester for the rest of his career, but his mindset has changed and his availability will inevitably attract interest from major forces such as Real Madrid and Barcelona. Manchester City may feel they have an outside chance of capitalising on what has gone wrong for him at Old Trafford, although their chances are undermined by the fact they already have a huge task ahead of them bringing down their wages to prevent Uefa banning them from European competitions under financial fair-play rules.

The full details are not yet clear but the underlying fact is that Rooney now feels that his working relationship with Ferguson has suffered potentially irreparable damage in the fall-out from tabloid allegations about the striker's private life, coinciding with a dramatic loss of form and a growing sense that the most successful manager in the business has taken a hard-line approach with his player.

Rooney has lost his place in the team, with Ferguson citing a supposed ankle injury, and the 24-year-old felt sufficiently emboldened last week to contradict his manager's version of events and make it clear he has not missed a single training session – and was, in essence, dropped for other reasons.

What has not been established is whether these events have coincided with a disagreement about the amount of money he expected to earn in a new contract. United had been willing to make him the highest earner at the club, with a weekly salary of £150,000, and the club's chief executive, David Gill, had stated several times earlier in the year that the matter would be resolved as soon as Rooney was back from the World Cup.

That now appears to have backfired on Gill, with the message already conveyed to senior figures at Old Trafford that Rooney is now counting down his days at the club. There remains a suspicion it might be a part of the negotiating process, but the Guardian has been informed that, for now at least, Rooney's mind is made up and that he and his family are already contemplating where next to take his career.

If that remains the case, it threatens to be an even more devastating blow to the club than Cristiano Ronaldo's £80m transfer to Real Madrid last year given that it was widely known the Portuguese would eventually move to the Bernabéu. Rooney has always given the opposite impression, immersing himself in the fabric of the club, and now appears to be on the brink of being one of the few players to leave against their wishes. Ferguson has moved on some great footballers, including Ruud van Nistelrooy and David Beckham, but always prides himself on players not being sold unless he says so; in this case it seems clear that the decision was made by Rooney first.

With United desperately trying to keep the matter in-house, Ferguson's views on the subject are not clear, other than he is known to be alarmed and angry about the headlines that Rooney has attracted because of his alleged relationship with a prostitute, Jennifer Thompson and mostly his faltering performances on the pitch. Some United fans will be angry that Rooney seems to have portrayed himself as the victim when it could be argued that he has brought these troubles on himself, but a rift has clearly developed between player and manager.

Rooney has been in the worst form of his professional life for the last seven months, without a goal in open play since the first leg of the Champions League quarter-final against Bayern Munich in March. Most worryingly, he is showing few signs of emerging from his current slump. Nonetheless, he remains a hero to the United support and was regarded as the player who would help to ensure continuity and success once Ferguson, plus the likes of Ryan Giggs and Paul Scholes, had retired.