A vast army of protesters has marched through central London to demand a new referendum on Britain’s departure from the European Union.

Organisers want the public to have a final say on the British government’s Brexit deal with the EU, arguing that new facts have come to light about the costs and complexity of Britain’s exit from the bloc since Britons voted to leave in 2016.

Some 150 buses ferried thousands of activists from across the country to the British capital for Saturday’s march, and organisers estimated that over 500,000 took part.

“What’s clear is that the only options on the table now from the prime minister are a bad Brexit deal, or no deal whatsoever,” London Mayor Sadiq Khan, who joined the People’s Vote March, told the BBC.

“That’s a million miles away from what was promised two and a half years ago.”

Prime Minister Theresa May, the leader of Britain’s Conservatives, has ruled out another public vote on the subject.

Britain is scheduled to leave the EU on March 29, but negotiations over the divorce have been plagued by disagreements, particularly over the future border between the Republic of Ireland and Northern Ireland.

It will be the UK’s only land border with the EU after Brexit, for Ireland is part of the EU and Northern Ireland is part of the UK.

There are also growing fears of a “no-deal” British exit, which could create chaos at the borders and in the EU and the British economies.

May, speaking at an inconclusive EU summit in Brussels this week, said she would consider having a longer post-Brexit transition period – one that could keep Britain aligned to EU rules and obligations for more than two years after its March departure.

Pro-Brexit politicians in Britain, however, saw it as an attempt to bind the country to the bloc indefinitely.

-AAP