Boris Johnson could be stopped from becoming prime minister even if he is elected as the new Conservative leader, two of Britain’s leading constitutional experts have said.

With Tory MPs threatening to withdraw support for the party under his stewardship, Johnson is warned that he could be prevented from ever entering Downing Street should it become clear he cannot command a majority in the House of Commons.

It is the latest sign of the parliamentary crisis that could face Johnson upon his election. It risks involving the Queen in politics and could pull Theresa May back into the Brexit impasse; as the incumbent prime minister, she will be key in recommending to the palace who should be called to form the next government.

Johnson’s legitimacy would be challenged if just a handful of Tory MPs declare that they could not support his administration, according to professors Robert Hazell and Meg Russell from the constitution unit at UCL, University of London. With May already struggling with a tiny working majority, two Tories – Dominic Grieve and Ken Clarke – have already indicated that they would not be able to support an administration that would leave the EU with no deal, which Johnson is willing to do.

The professors conclude Johnson would “not necessarily” become prime minister should he beat Jeremy Hunt and become Tory leader at the end of July. The cabinet manual, which covers changes in government, does not deal with the unusual circumstances that have been created in parliament by the Brexit dilemma.

“The key test is whether the Conservatives’ new leader is able to command the confidence of the House of Commons,” they write. “Whether the new Conservative party leader can command parliamentary confidence is clearly in some doubt given comments from Conservative MPs that they may not be able to support the new government. The government only has a majority of three, including the DUP, so only a very few rebels is enough for it to lose its majority.

“One possible scenario is that a group of Conservative MPs is so concerned about the winning candidate that they declare their withdrawal of support immediately the result of the leadership contest is known – ie, before the new PM is appointed. This would pose a serious dilemma for the Queen and those advising her, because it would not be clear that the new Conservative leader could command confidence.”

This would pose a serious dilemma for the Queen and those advising her Professors Robert Hazell and Meg Russell

One senior Tory said it was simply a statement of reality that Johnson would struggle to form an administration. However, he predicted that Johnson would be allowed to do so at the end of July, but would be likely to face a no-confidence vote when MPs returned from summer recess. “It’s very probable that he will be able to set up an administration and the crunch in terms of his legitimacy won’t come until September,” he said.

In their analysis, Hazell and Russell conclude that the Queen could make the new Tory leader a “provisional appointment” as prime minister, conditional on him demonstrating he has the confidence of enough MPs. “Alternatively, Theresa May could remain in place and facilitate a process in parliament to demonstrate that the winning candidate – or indeed an alternative candidate – can win a confidence vote, before recommending that person to the Queen.”

It is currently unclear who an alternative candidate could be. Labour would not have the numbers to form a government, so some kind of unity-government figurehead could be an option. Many MPs on all sides now believe that the crisis will end in an autumn general election, either called by Johnson or forced upon him MPs unwilling to leave the EU with no deal..