London, Mar 23 (IANS): British Prime Minister Theresa May has told MPs that there might not be a third vote on her Brexit deal next week if there was insufficient support for it to pass.

If it does not pass and no alternative plan is put forward, the UK is set to leave the European Union (EU) on April 12. And if it passes, the bloc has agreed to extend the Brexit deadline until May 12, the BBC reported.

In a letter to all MPs on Friday night, May offered to talk to them over the coming days "as Parliament prepares to take momentous decisions".

She said there were now four "clear choices", which were approving her deal next week; asking for another extension before April 12 which would mean the UK would have to take part in elections for the European Parliament; revoking Article 50 or cancelling Brexit; and leaving the EU without a deal.

May also confirmed in her letter that the government would change the law to officially change the UK's initial departure date of March 29 from the EU.

Meanwhile, supporters of another referendum will march through central London on Saturday.

Campaigners say they expect hundreds of thousands of people to attend the 'Put It To The People' demonstration, which will be addressed by Labour's deputy leader Tom Watson, Scotland's First Minister Nicola Sturgeon, London Mayor Sadiq Khan and others.

Sturgeon has said that now was "the moment of maximum opportunity" to avoid a no-deal Brexit.

A petition on Parliament's website calling for Brexit to be cancelled by revoking Article 50 has attracted nearly four million signatures.