British MPs voted Wednesday to force Prime Minister Theresa May to quickly set out an alternative plan for Brexit if she loses a crucial vote on her EU withdrawal deal next week, in a second parliamentary defeat in 24 hours. The House of Commons voted to reduce the time the government has to outline a "plan B" from 21 days to three if, as expected, lawmakers reject the Brexit agreement in a vote on Tuesday. Members of May's own Conservative party led the revolt amid fears that the current timetable takes Britain too close to crashing out of the European Union on March 29 with no deal at all. It was the second setback in as many days for the prime minister, after MPs voted late Tuesday to deny the government certain taxation powers in a no-deal scenario -- another attempt to avoid such an outcome. Speaking to MPs earlier, May said: "The only way to avoid no deal is to vote for the deal." But her spokesman added: "If that were not to take place... we would respond quickly and provide certainty on the way forward." Opposition Labour spokesman Keir Starmer said MPs could then start debating alternatives to her plan -- and raised the prospect of delaying Brexit. "I actually genuinely think we can't do it on March 29 this year. It's simply not viable, for so many practical reasons," he told the Commons. "So we're going to have to look at what are the available options that realistically are still on the table." - 'Cosmetic' changes - The parliamentary manoeuvres came at the start of five days of debate on the deal May struck with the EU last November, which has been strongly opposed from lawmakers on all sides. She set out further clarifications she hopes will win over her own Conservative MPs and Northern Ireland's Democratic Unionist Party (DUP), which props up her government in parliament. The most controversial element of the agreement is the "backstop" arrangement, which could see Northern Ireland continue to follow certain EU trade rules after Brexit to avoid border checks with Ireland. The government promised a greater say by British politicians, including devolved politicians in Northern Ireland, over the operation of the backstop, as well as guarantees on the free flow of trade between the province and mainland Britain. But DUP deputy leader Nigel Dodds said the proposals were "meaningless and cosmetic". May is also seeking further assurances on the temporary nature of the backstop from European leaders, which she hopes to deliver before the vote next week -- but they insist they will not reopen the deal. The prime minister has already postponed the vote once -- in December -- to avoid defeat by MPs, but insists next week's will go ahead. - No deal fears - May has warned that if MPs reject her deal, Britain would still leave the EU on March 29, with or without any new arrangements. Britain triggered two-year legal withdrawal process in March 2017 that will run its course unless London delays or revokes it. One of her own ministers this week raised the idea of a delay, which has been the source of speculation for weeks in Brussels, but May has rejected the idea. Both this week's revolts were driven by Conservative MPs seeking to prevent a "no deal" scenario, which they fear would cause huge economic disruption. Late on Tuesday, opposition MPs and more than a dozen former Conservative ministers voted to restrict the government's taxation powers in a "no deal" scenario. The government said it was an "inconvenience" but would not stop Brexit preparations. Wednesday's amendment, which passed by 308 votes to 297, has more weight. It states that if May loses the vote next week, she must explain her next steps within three parliamentary sitting days -- by the latest, Monday January 21. She would then have a further seven days to put a motion to the House of Commons, which MPs could amend to try to direct the government's strategy. There was a major row in the Commons over whether Wednesday's amendment could even be put to a vote, with Speaker John Bercow apparently disregarding the advice of his own clerk that it could not. Several Conservative MPs accused Bercow of being biased, and there was speculation of an attempt to remove him from his post. Labour has said it will seek a confidence vote in the government if it loses next week, and is also likely to press for a general election.