Prime Minister Theresa May has warned dissenting British MPs to approve her unpopular deal to leave the European Union or risk no Brexit at all.

In a last ditch appeal on the eve of the historic vote on her Brexit deal, Ms May argued that Parliament was more likely to prevent Brexit than allow the UK to leave without a deal.

Speaking to workers at a factory in Stoke-on-Trent, a part of the UK that voted heavily in favour of leaving the EU, the Prime Minister said if lawmakers voted down her deal, it would result in “catastrophic harm” to trust in politics.

“The only deal on the table is the one MPs will vote on tomorrow night,” Ms May said.

“You can take no-deal off the table by voting for that deal. If no deal is as bad as you believe it is, it will be the height of recklessness to do anything else,” she added.

Ms May’s plea came as deputy whip, Gareth Johnson, become the latest member of the government to quit his job over the Brexit deal, saying in his resignation letter to the PM that it would be “detrimental to our nation’s interests”.

“The time has come to place my loyalty to my country above my loyalty to the government,” he said.

Mr Johnson was the latest senior Conservative to quit over the deal. Photo: Gareth Johnson

With less than 24 hours left until the vote, Britain’s future path to Brexit is still uncertain with Parliament expected to heavily vote down Ms May’s deal.

Possible outcomes include a last-minute deal, a disorderly exit, a new referendum or remaining in the bloc.

The opposition Labour Party is also likely to launch a motion of no confidence in the government if the Brexit vote is heavily defeated.

EU earlier issued a letter to the British government spelling out a series of reassurances on the contentious “Irish backstop”, an insurance policy to prevent the return of border controls between the British province of Northern Ireland and the Irish republic.

The backstop is designed to preserve the open border between EU-member Ireland and the UK governed Northern Ireland, which many MPs believe hands control to Europe.

In the EU letter, President of the European Council Donald Tusk and EU Commission President Jean-Claude Juncker said Brussels “does not wish to see the backstop enter into force” but if it did, it should only be temporary”.

If, as expected, Ms May’s Brexit deal is voted down, the government will have three working days to come up with a “plan B” alternative to be put to a further vote on January 21.

In the meantime, Ms May could head to Brussels to try to get further concessions from the EU.

-with AAP