Elections for new Euro-MPs must be held if MPs delay Brexit, pressure group Leave Means Leave has said.
A fresh round of European Parliament elections will take place across Europe on May 23, with the new MEPs due to be sworn in on July 1.
Under the Article 50 process, Britain is due to leave the EU on March 29 regardless of whether an agreement is in place.
But if Brexit is delayed and the UK is denied the right to elect new MEPs, the hardline Eurosceptic group says ministers would “deny democracy” to the British people.
Lawyers acting for the group have written to de facto deputy PM David Lidington to ask for assurances by Friday March 18, ramping up the pressure on Theresa May.
The meaningful vote on the PM’s Brexit deal will happen on Tuesday, but it is widely anticipated MPs will reject it.
Following that, the Commons will vote on whether to rule out a no-deal Brexit on Wednesday and, on Thursday, whether to agree an extension of the Article 50 deadline.
MPs fear a no-deal Brexit would unleash chaos at the border and cause food and medicine shortages. They may try to stop the UK from crashing out by forcing May to seek an extension.
John Longworth, chairman of Leave Means Leave, whose organisation favours crashing out, said: “We are determined to challenge the government and EU if they attempt to deny the democratic right of the people of the UK to be represented in the EU Parliament.
“It would be quite wrong that we should be trapped in the EU but with no say. It would be remarkable if Parliament were to try to deny democracy on this matter just as they have been trying to do on the matter of Brexit.”
It comes as the government is making a last-ditch bid to strike a deal with Brussels on the controversial Northern Ireland backstop, with Attorney General Geoffrey Cox set to update legal advice on the agreement in a bid to win over the DUP and Tory Brexiteers.
At the last European elections in 2014, UKIP, then led by Nigel Farage, came top of the poll, winning 24 seats and 27% of the popular vote.
Richard Tice, founder and vice chairman of Leave Means Leave, said: “The government and the British and EU establishments cannot be trusted. They may try to extend Article 50. A short extension serves no purpose not least because the European Parliament goes into recess in mid-April.
“If the PM or Parliament try to extend beyond the new Parliament i.e. the 2nd of July, the EU constitution will require that the UK hold elections for MEPs.
“If they try to extend beyond the May election date but short of July 2, we believe this will be a ploy to avoid the EU elections, but with the intention of further extending once the date has passed. The government cannot be trusted not to further extend. “