Dominic Raab was branded a “dictator” by a rival camp in the Conservative leadership race after the leading Brexiteer suggested that he would be prepared to shut down the UK Parliament to ensure Brexit happened.

On the eve of Theresa May stepping down as Tory leader and ahead of Monday’s start to the contest to succeed her, the former Brexit Secretary was also warned by constitutional experts that any such suspension risked dragging the Queen into the bitter Brexit battle.

During a hustings event in the Commons before the One Nation Tory group, Mr Raab sparked outrage when he refused to rule out proroguing Parliament - bringing an end to the session - to prevent MPs blocking the UK's withdrawal from the EU on October 31.

But critics said the approach would be "a coup against Parliament" and Matt Hancock, the UK Government’s Health Secretary and another Tory leadership contender, called on all his rivals to rule it out.

A source close to Sajid Javid, the Home Secretary, who is also vying to succeed the Prime Minister, noted: "He firmly believes his honest and credible deal on Brexit can command support of the Commons and the EU. But he wants to be PM; not a dictator."

An angry John Bercow, the Commons Speaker, also intervened, insisting he would not allow MPs to be sidelined.

"Parliament will not be evacuated from the centre stage of the decision-making process on this important matter. That's simply not going to happen," he declared.

Parliamentary expert Hannah White, deputy director of the Institute for Government thin-tank, said any move to prorogue in these circumstances "would amount to a coup against Parliament and risk bringing the Queen into a terrible conflict".

She explained: "The implication of Raab's refusal to rule out this strategy is that he thinks it would potentially be legitimate to suspend Parliament, not simply to let the Article 50 clock run down but to prevent MPs from making a decision he knows they would want to take.

"This would be extremely controversial, particularly in these circumstances because the Government does not have a majority.

"Asking the Queen to give effect to this strategy would draw her into a massive political debate; something which Number 10 and the Palace are normally at great pains to avoid."

Dr White suggested that Buckingham Palace "would look for ways to limit the Queen being drawn into the process", possibly by delaying long enough to allow Parliament the opportunity to prevent prorogation.

One way, she noted, could be through a no-confidence motion to bring down the Government.

"The effect of attempting to prorogue Parliament to pursue a no-deal policy to which it has thus far been firmly opposed would likely be to prompt an election. That may even be the intention," added Dr White.

A source close to Mr Raab said: “His point is that if Parliament won’t rule out what options they will use, then why would the executive rule out options?"

A supporter of the former Cabinet minister pointed out that ruling out a no-deal Brexit or seeking an extension beyond October 31 meant "weakening our negotiating leverage in Brussels".

He added: "Dominic made clear that he would use every lever of the executive to ensure that the UK can leave the EU at the end of October."

But the suggestion of prorogation caused a backlash among Conservative colleagues.

Mr Hancock wrote to his rivals, saying: "To suspend Parliament explicitly to pursue a course of action against its wishes is not a serious policy of a prime minister in the 21st century.

"What kind of message would this send around the world about our values when so many have given so much for the rights of democratic freedom?"

Rory Stewart, the International Development Secretary and another leadership contender, said proroguing Parliament would be "unlawful, undemocratic, and unachievable" and "the idea itself is profoundly offensive to our liberty, constitution and traditions".

Amber Rudd, the Work and Pensions Secretary and a leading player in the One Nation group, said: "It's outrageous to consider proroguing Parliament. We are not Stuart kings."

Meanwhile, MPs are expected to be given the chance to decide whether Mrs May's successor as Conservative leader should face an immediate confidence vote.

Downing St said the Commons was expected to be sitting when the new PM was appointed in July. Its intervention came after Commons Leader Mel Stride refused to be drawn on whether the House would be sitting when the new premier took office.

Labour said it was “vital” MPs had the opportunity to test the stability of the Government under its new leadership.

Elsewhere, the Democratic Unionist Party, which won £1 billion for Northern Ireland when it entered into a confidence and supply agreement two years ago, has made clear it will seek “similar resources” when the deal is up for renewal this summer.

In other developments:

*Jeremy Hunt, the Foreign Secretary, said he would use the UK's no-deal Brexit "war chest" of £26bn to copy Donald Trump's tax-cutting agenda to "turbo-charge" the economy;

*Andrea Leadsom, the former Commons Leader, said she would declare a climate emergency if she became PM as she said tackling climate change was a "massive opportunity" for the UK to "lead the world";

*candidates will take part in a BBC TV debate at 8pm on June 18, just hours after finding out if they have survived the second round of voting by their colleagues and

*Treasury Minister Jesse Norman announced he had decided not to throw his hat in the ring.