Mike Ashley, the Newcastle United owner, has been urged to allow manager Rafa Benitez to strengthen the squad this month, regardless of whether he is able to sell the club, in an open letter signed by all the major supporter groups on Tyneside.

Ashley is still hoping to find a buyer for a club he has owned since the summer of 2007, but the initial confidence that a deal was close is fading and there now appears to be little hope of a takeover taking place before the end of this month.

Although there are believed to be four interested parties who have approached Ashley’s representatives, none have managed to push through a deal and at least two of them are understood to be struggling to match the asking price.

In turn, there is a suspicion that Ashley could use talk of a takeover as an excuse not to invest in the team and to distract attention from the fact Benitez is desperate for new signings to be made.

Ashley has already taken around £10m out of the club this season in order to repay a short-term loan he gave the business during Newcastle’s season in the Championship and it is thought he is due to receive another larger repayment this month.