While May was addressing her lawmakers, MPs in the main chamber debated eight other Brexit options, ranging from leaving abruptly with no deal to revoking the divorce altogether.

While there was no majority support for any, the option calling for a referendum on any departure deal, and another suggesting a UK-wide customs union with the EU, won more votes than May's deal did two weeks ago.

Oliver Letwin, a Conservative former cabinet minister who led parliament's unusual power grab, said it had been expected that none would immediately win a majority, and lawmakers would have another chance on Monday.

The uncertainty around Britain's most significant political and economic change since World War II has left allies and investors aghast. Sterling dipped after Wednesday's events.

If May's deal were to pass, her office said there would be a contest to replace her after May 22, when Britain would leave the EU.

She would also become the fourth Conservative prime minister in a row to have fallen foul of divisions over Europe within her centuries-old party, following David Cameron, John Major and Margaret Thatcher.

Most voters think the Brexit negotiation has been handled badly and there may now be a slight majority for staying in the EU, recent polls suggest. Many Conservative MPs say May herself has caused the chaos by not negotiating harder with the EU.

"It was inevitable and I just feel she's made the right decision. She has actually read the mood of the party, which was a surprise," said Conservative lawmaker Pauline Latham.

Reuters