The government's credibility "is running out", a Tory former attorney general and Bucks MP has warned.

Dominic Grieve QC issued a strongly worded broadside against the Prime Minister in the Commons, arguing that unless she "stands up and starts doing something different, we are going to spiral down into oblivion".

The senior Conservative said: "When...the Prime Minister came to the despatch box today at PMQs, I confess I think it was the worst moment I have experienced since I came into the House of Commons.

"I have never felt more ashamed to be a member of the Conservative Party or to be asked to lend her support.

"She spent most of her time castigating the House for its misconduct, at no stage did she pause to consider whether is it in fact the way she is leading this government which might be contributing to this situation."

He added: "I have to say I could have wept, wept to see her reduced to these straits and wept to see the extent to which she was now simply zig zagging all over the place, rather than standing up for what the national interest must be."

The MP for Beaconsfield said the "underlying integrity which one hopes one will continue to see from Government even in difficult circumstances now seems to be fast running out and that troubles me very much".

He spoke of a wider pattern of the "complete disintegration" of collective responsibility in Government, adding: "We have ministers coming to the despatch box and saying entirely contradictory things.

"We have ministers publicly disassociating themselves from Government policy and staying in post."

He said: "We have ministers who come up to one in the corridors and acknowledge that the situation is very serious and they disagree with what the Government is doing continuing to serve in a cabinet with which they apparently fundamentally disagree."

He added: "You don't meet that challenge by ducking and diving and avoiding and having a galaxy of ministers appear at the despatch box and say contradictory things.

"You've got to face up to your responsibility, and rather than coming along and showing contempt for this House, actually trying to engage with it and making use of what this House can do pretty well which is debate issues in a rational way and which in itself by a process of debate might lead to a reasonable outcome."

Mr Grieve warned the UK was "in danger of crashing out" with no deal, asking if MPs were prepared to look at alternatives when "it seems so apparent that the deal itself is going to be rejected".

He said: "Just browbeating this House is going to serve no purpose at all."

He added: "If we do not do it, one has to ask oneself the question, what is the purpose of this Government? What is it doing? How is it furthering the national interest?

"How is it contributing to the quiet good governance which I think most people in this country want."

He added: "We really are, I'm sorry to say this, at the 11th hour and 59th minute, the Government's credibility is running out, trust in it is running out and unless ...the Prime Minister by some great exertion of will and she has plenty of will and plenty of robustness, stands up and starts doing something different, we are going to spiral down into oblivion and the worst part of it all is that we will deserve it."