Juan Mata's £37million move to Manchester United almost collapsed - when Chelsea told him he could join anyone but the Old Trafford giants.

Mata had a verbal agreement with Blues boss Jose Mourinho that if he wasn’t a first-team regular by Christmas, then the London club would let him go during the January transfer window.

Before Christmas, Chelsea put in writing permission for Mata’s representatives to broker the Spain midfielder's transfer.

But he was stunned to discover in this document that the exit strategy came with the caveat that he could move to any club willing to meet Chelsea’s £37m valuation – except United.

Sunday Mirror Sport ­understands that their stance was prompted by United’s refusal to sell them Wayne Rooney last summer.

United were furious at Chelsea’s aggressive approach for the England striker during the fall-out between the England star and outgoing boss Sir Alex Ferguson.

Mourinho publicly declared that Rooney was his top target and repeatedly claimed that he believed they would get their man in August. However, United stood firm.

To prevent Rooney being even discussed within the Mata deal, United tabled their bid through the Spaniard’s representative, Colin Pomforde. Chelsea used Kia Joorabchian as their nominated agent.

United have succeeded in ­stopping Rooney being linked again with Chelsea. In fact, the 28-year-old is now in line for a proposed, new ­four-year contract worth more than £300,000-a-week.

The war of words surrounding Rooney last summer led to a complete breakdown in the ­relationship between United and Chelsea.

Chelsea were even prepared to sell Mata to title rivals Manchester City and Arsenal.

However, they had to soften their hardline stance when it became clear that United were the only club willing to pay £37m for the Spanish star.

Mata also made his anger clear, because he wanted the opportunity to push his claim for a a World Cup place after falling out of favour with manager Mourinho.

United first enquired about Mata last July, but were told Chelsea would only do business if the deal allowed Rooney to move in the opposite direction. That part-ex proposition killed the proposal stone dead.