LONDON (Reuters) - Pro-European Union campaigners in Britain have set out for the first time their preferred path for how parliament could force the government to call a fresh vote on Brexit, arguing that there is still time for another referendum.

Pro-EU supporters attend a joint political rally by Best for Britain and the People's Vote campaign in London, Britain, December 9, 2018. REUTERS/Peter Nicholls

The future of Brexit remains deeply uncertain - with options ranging from a disorderly exit to another referendum - because British lawmakers are expected on Jan. 15 to vote down the deal that Prime Minister Theresa May struck with the EU in November.

May has repeatedly rejected the idea of a second referendum on leaving the EU, but the campaign for a so-called “People’s Vote” on the deal that May has agreed, has won support from some in parliament.

If May’s deal is voted down next week, ministers have to say in a parliamentary motion how they plan to proceed within 21 days.

The People’s Vote campaign said in a report that lawmakers should amend that motion by calling for another referendum. This would happen around the middle of February.

Britain would then be forced to ask for an extension to its timetable for leaving the EU to allow enough time for another referendum campaign, which may take around four months.

“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 campaign group said in the report. “We believe the only credible way forwards for (lawmakers) will be to hand the decision back to the people.”

Turning Brexit upside down would mark one of the most extraordinary reversals in modern British history and the hurdles to another referendum remain high. Both major political parties are committed to leaving the EU in accordance with the 2016 referendum.

The path to a new referendum is reliant on May, who does not have an outright majority in parliament, failing to win over skeptical lawmakers within her own party.

Extending the divorce beyond March 29 would require the unanimous agreement of EU heads of state and government in the European Council.

But the People’s Vote says that if the United Kingdom asked to delay Brexit so it could hold another referendum, the other EU countries would be likely to agree.

It said a new vote should ask a binary question such as whether voters wanted to accept the government’s leaving deal or stay in the EU, or another form of Brexit versus staying in.

The campaign group said that if there is another referendum, there is a strong case for EU citizens living in the United Kingdom, expatriate citizens who have lived outside the UK for more than 15 years and young people aged 16 and 17 to be given a vote.