George Osborne has been warned that a "significant number" of Tory MPs are ready to throw out plans for a £1.3 billion a year cut in disability benefits.

MP Andrew Percy said the squeeze on personal independence payments (PIPs) sent out the wrong message and has organised a letter to the Chancellor from concerned backbenchers urging a rethink.

Read more: John McDonnell urges Conservative MPs to rebel over 'new low' in disability cuts

Welfare officials expect up to 640,000 people to be affected by tighter criteria, brought forward after a review by health professionals found people were being awarded points for aids and appliances already in homes or provided by the NHS and councils.

Ministers insist that overall spending on disability benefits is going up and the changes are needed to make sure the cash is better targeted to those in most need.

But Mr Percy said it was clear the shift was aimed at hitting spending targets, not improving support, and made clear that the Government was on course to suffer defeat in the Commons if it sought to push the change through.

"There is an argument within some of the criteria but we have to view it for what it is primarily, which is a fiscal measure," he told BBC Radio 4's The World At One.

"It is not about welfare reform or necessarily wholly about getting better value for money. This is about the welfare cap."

He would not disclose how many had signed the letter, or its contents, but added: "The Government has a very small majority so you don't need very many for this to be a problem of parliamentary arithmetic.

"It is fair to say the numbers on this who have expressed concern are very significant indeed."

While some of the arguments used to justify the cut had merit, politically the issue for politicians on the doorstep was "the message it sends and the numbers it will affect", he said.

He wrote on Twitter: "If I can be honest, I'd rather have a penny or two on fuel if it protects PIP."

Mr Osborne told the BBC he was "always happy to listen to proposals about how to improve on that".

"But we have got to control our disability budget and make sure help goes to the people who need it most."

Welfare minister Lord Freud said that "on the present trajectory" PIP costs were moving up to £12 billion, compared with an earlier expectation of £9 billion.

"So we are reducing a rapid growth and adjusting how we get to PIP because clearly we are getting much higher figures than was originally expected through the use of those aids and appliance measures."

Shadow chancellor John McDonnell urged MPs across the House to force a rethink.

"Even for a Chancellor who has repeatedly cut public spending on the backs of those least likely or least able to fight back, this represents a new low," he told the Commons.

"I believe it's morally reprehensible."

Mark Serwotka, leader of the Public and Commercial Services union, said: "It is sickening to see George Osborne targeting disabled people with £4 billion of cuts to fund tax giveaways to high earners and businesses, in pursuit of his Tory leadership ambitions.

"Disabled people should not have to suffer because of the failings of DWP (Department for Work and Pensions) and the private providers. We will continue to fight this plan until it is scrapped."

Downing Street said discussions would continue with MPs and disability groups but the change was about ensuring that taxpayers' money goes to those most in need.

Asked what David Cameron's message to Tory rebels would be, the Prime Minister's official spokeswoman said: "He would say to them this is about how we ensure that taxpayers' money is going to those most vulnerable, most in need, so that we can focus support on those who need it."

She added: "We consulted last year on how best to support the most vulnerable. We have published our response and we are now taking this forward, continuing discussions with MPs across the House and with disability groups."