Theresa May should reform controversial rules restricting the voting rights of Scottish MPs before Brexit creates a parliamentary crisis, according to a new report.

Academics today warn that changes to the system of “English Votes for English Laws” (Evel) are needed to give them greater legitimacy across the UK,

Alterations are also required to strengthen the influence of English MPs at Westminster.

The Prime Minister risks a parliamentary crisis if she fails to act, they suggest.

Professor Michael Kenny from Queen Mary University of London said: “So far, this system has been operating out of political sight but Evel may well turn into a headache for Theresa May.

“But what would happen if a majority in the Commons were to vote down legislation that followed on from a ‘hard Brexit’ deal which only applied to England and had the support of a majority of English MPs?

And what if measures such as the government’s proposals to introduce more grammar schools in England are blocked by a Commons majority that includes the SNP and other non-English members? "

The report, Finding the good in Evel, published today by the Centre for Constitutional Change with the support of the Economic and Social Research Council and the Constitution Unit at UCL, also found that MPs described Evel as an "additional source of tension between the nations of the UK".

The researchers also warn that the system risks politicising the role of the Commons Speaker.

The current Speaker John Bercow has told MPs that he will not set out his reasons for judging a Bill to be England-only.

But the report urges him to reconsider, saying that greater understanding of why a Bill is designated England-only could add greater legitimacy to the process.

Recommendations for reform include that Evel should be more accurately renamed 'English Consent to English Laws' (Ecel).

The system currently strips Scottish MPs of the right to vote at certain stages of Bills dubbed "England only".

Earlier this month peers warned that Evel risked damaging the Union.

David Cameron faced accusations of betrayal after he announced plans to introduce Evel less than an hour after Scotland voted to remain within the United Kingdom in 2014.

Critics pointed out that the policy had not been included within the "Vow" offering more powers to Scotland and signed by the prime minister before polling day.

Outraged Scottish MPs complained that the system would create 'two tiers' of MP.

But the Conservative Government argued that the previous system was unfair because it allowed Scottish MPs vote on issues that did not directly affect their constituents.

Evel is designed to solve the 'West Lothian Question' coined by the former labour MP for the area Tam Dalyell before devolution.

Famously, Mr Dalyell asked how it could be right that he would be able to vote on education policies that would affect Blackburn, Lancashire but not Blackburn, West Lothian.

The SNP called on the Conservative Government to scrap not reform Evel.

SNP MP Kirsty Blackman said: "Evel was motivated by the political interests of the Tory party. This dog’s breakfast of a procedure has relegated Scottish MPs, it has seriously risked politicising the Speaker, and it has diminished the ability of Scottish MPs to influence legislation which might have funding implications for Scotland.

"We welcome the fact that SNP pressure has led to the UK Government reviewing the procedure but with countless experts, parliamentary committees and now even backbench Tory MPs lining up to brand the procedure as simply not fit for purpose – the Government must take this opportunity to scrap Evel once and for all.”

Scotland's only Labour MP Ian Murray said: "This just shows that, far from being strong defenders of the union as they promised, the Tories have out it at risk at every turn since 2014.

“David Cameron's disastrous EU referendum and Evel plan has been a constitutional wrecking ball which has only fanned the flames of nationalism.

“And let’s not forget that the SNP used to be big supporters of Evel. This is just another example of their abject hypocrisy and willingness to do or say anything in their pursuit of independence.”