British MPs on Friday rejected Prime Minister Theresa May's EU divorce deal for a third time, opening the way for a long delay to Brexit -- or a chaotic "no deal" withdrawal in two weeks.

Pro-Brexit demonstrators wave Union flags as protesters gather in Parliament Square in central London on March 29, 2019. (Photo: AFP)

The pound sank as lawmakers defied May's plea to end the deadlock that has plunged Britain into a deep political crisis, defeating her withdrawal agreement by 344 votes to 286.

It is yet another blow to a prime minister who has all but lost control of her government and the Brexit process -- particularly after she offered to quit if MPs backed the deal.

Britain had been due to leave the EU on Friday, the long-heralded March 29 "Independence Day", but faced with deadlock in parliament, May asked European leaders last week for a little more time.

She now faces having to return in the coming days to explain what happens next, after EU Council President Donald Tusk immediately called a Brussels summit for April 10.

The EU has set a deadline for April 12 for a decision, with two likely options: Britain leaves with no deal at all, or agrees a lengthy extension to allow time for a new approach.