With less than 80 days to go until Britain is set to leave the European Union, campaigners hoping for another Brexit referendum have laid out their preferred path for a fresh vote.

Britain is scheduled to leave the European Union on March 29 but, as the country's deadline for departure draws ever closer, what will actually happen on that day remains far from certain.

Embattled Prime Minister Theresa May is still struggling to overcome deep opposition to her proposed withdrawal plan, opening up a range of possibilities from a disorderly exit to the whole process being scrapped altogether.

"Nobody has come forward with a proposal that could secure a majority in the present circumstances. The blunt reality is that such a proposal does not exist," the People's Vote campaign group said in a report published Wednesday.

"We believe the only credible way forwards for MPs (Members of Parliament) will be to hand the decision back to the people."

An all-important parliamentary vote on the draft withdrawal agreement is due to be held on Tuesday next week.

Westminster must decide whether to accept May's plans for a structured exit and relatively close economic ties or reject it and spawn uncertainty over the country's next steps.