A referendum on the Brexit deal would take at least six months to organise legally, making it very difficult to have a second vote before the UK is scheduled to exit the EU on 29 March next year, constitutional experts have said.

As EU leaders including the Czech prime minister, Andrej Babiš, called on Theresa May to change firm government policy and put a vote to the people, academics said there was not enough time if article 50 is enacted as scheduled.

There are indications that a delay to the enactment of article 50 could be acceptable to the EU, but without this agreement time stands in the way of a second referendum, experts believe.

“It is just possible to hold one within six months, but the shorter the timescale, the higher the chance of the question or other aspects of the referendum being challenged over their legitimacy,” said Prof Robert Hazell of the constitution unit at the department of political science, University College London.

Fresh legislation, testing of the question by the Electoral Commission and a 10-week regulated period for a campaign are all required before a referendum can take place, he pointed out.

David Cameron’s Brexit referendum took just over a year to get to the ballot box.

The UCL researcher Jess Sargeant, who is about to publish a report on a second referendum with Alan Renwick, the deputy director of the constitution unit, said that even if all elements of the process went smoothly and speedily, “the total period from start to finish would be 24 weeks”.

The first hurdle is the requirement for primary legislation, which would be highly contentious and require due process in parliament.

“With political will, legislation can be rushed through on significantly reduced timescales,” says Sargeant in a blogpost on referendum timetables.

But, she warned: “The government’s narrow majority, dependent on DUP support and subject to internal party divisions, makes the situation highly unpredictable.”

Hazell pointed out that if such a law was passed, then the Electoral Commission would have to advise on the “intelligibility” of the referendum question, a process that usually takes 12 weeks.

The commission would than designate lead campaigners to both sides, which adds more time.

Time taken to hold referendums. The poll on introducing an ‘alternative vote’ system for elections was the shortest, at nine months. Illustration: Constitutional Unit, University College London

And finally, under the Political Parties, Elections and Referendums Act (PPERA), a 10-week campaign period is specified.

Five referendums have been held under PPERA, three of which were enabled through primary legislation, including the North East England assembly ballot, the alternative vote referendum in 2011 and the 2016 Brexit vote.

The AV referendum, which took the shortest time, took nine months. The Brexit referendum took just over a year from start to finish.

May said her government would never accept a second referendum on Brexit. But a report, published on Tuesday (pdf), and backed by the author of article 50, Lord Kerr, argues it is not too late to have one.

The report by campaigners for a second referendum, the People’s Vote, said one way to condense the timetable would be to “cut and paste” the legislation that was used for the 2016 referendum.

It also said that given the urgency of the matter, politicians could fast-track legislation through parliament to enable a swift vote.

UCL points out that with political will, the legislative process can be shortened. In 2005 the controversial Prevention of Terrorism Act (PTA) 2005 received royal assent within three weeks of its introduction.

The People’s Vote said politicians would e judged harshly if they denied calls for a second referendum on the grounds that it would be too difficult to arrange in the short time remaining.

“History will not, in our opinion, be kind to any politician who hides behind purely logistical arguments, legalese or arcane parliamentary procedure in order to deny people a vote on the outcome of these Brexit negotiations at such a fragile and crucial moment for our country,” said Kerr.