LONDON, July 19 (Xinhua) -- The pound sterling slumped in value Thursday as the fallout over the government's soft-Brexit plan intensified at Westminster.

In a blow for Prime Minister Theresa May, one of her Conservative MPs, Philip Davies, made public a letter he has submitted saying he has no confidence in her.

In the city, the pound dipped below 1.30 U.S. dollars sliding to a nine-month low against a trade-weighted basket of the leading currencies before stabilising at a fraction over 1.30, still representing a 0.5 percent loss. It was the third day in a row that had seen the value of the pound falling, with the political woes surrounding May and Brexit partly to blame, according to financial commentators.

Speculation among political commentators in London is that around 40 MPs are thought to have told the chairman of the Conservative backbench committee that they have lost confidence in May.

Under party rules if 15 percent of Conservative MPs submit confidence letters there would have to be a vote. If the figure of around 40 is accurate, it would only need a further eight to see May fighting to stay at 10 Downing Street.

As it is a secret process, nobody, apart from the chairman, knows how many letters in total have been sent in.

Davies decided to go public with his letter, which was published in the provincial Yorkshire Post newspaper.

In a letter to his constituents in Shipley, Davies said May's Brexit plan on trade was unacceptable.

He wrote: "Many people have told me they have lost trust in the PM (Theresa May) to properly and fully deliver the EU referendum result. It is with much sadness that I have to say that I have also lost trust in her to deliver the referendum result too.

"Failure to keep our promise to the electorate will almost certainly lead to the catastrophe of Jeremy Corbyn (Labour leader) becoming Prime Minister and I cannot sit back and allow that to happen," he said.

Davies added he had reluctantly come to the conclusion that he had no alternative but to send a letter asking for a vote of no confidence in the Prime Minister.

"This has not been an easy decision and I have agonised over it, but I know in my heart of hearts it is the right decision," wrote Davies.

A vote of confidence would not automatically bring down May's government, even if May lost the vote among her MPs.

But even calling a vote would harm the Conservative party and could pave the way for a snap general election.