Pressure was mounting on the Conservative chief whip Andrew Mitchell last night amid fresh signs that his MPs are losing confidence him. He has suffered the humiliation of pulling out of the party conference in Birmingham despite being the only Tory to represent a parliamentary seat in the city.

His ill-tempered outburst at police officers guarding the Downing Street gates – in which he was alleged to have called them "f***ing plebs" – undermined the conference preparations. Mr Cameron condemned his comments and has tried to draw a line under the affair.

However, several MPs are now understood to have written to Graham Brady, chairman of the backbench Tory 1922 committee, complaining that he is still in his post. One prominent backbencher told The Independent: "This business is doing us lasting harm. The word 'pleb' seems to have passed into common parlance."

Other sources suggest that some Tory constituency associations have contacted Conservative Central Office to warn that the controversy has hit the party on the doorsteps.

The test of Mr Mitchell's authority will come on Monday when MPs return to the House of Commons at the end of the party conference season. He was switched to the job – replacing Patrick McLoughlin – to impose greater discipline on a fractious parliamentary party, many of whose members have got into the habit of rebelling.

Mr Mitchell admitted directing a tirade at the police officers who refused to let him wheel his bicycle out of the main Downing Street gates and apologised for his comments. But he denied using the exact remarks attributed to him, notably the word "pleb". His defence left him in the position of contradicting the official police log detailing the incident. Labour has leapt on the episode, with shadow ministers repeatedly referring to it during their party conference in Manchester last week.