British MPs have voted 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.

It is the second parliamentary defeat for Mrs May in 24 hours.

The House of Commons voted to reduce the time the government has to explain its "plan B" from 21 sitting days to three if, as expected, MPs reject the draft Brexit agreement in a vote next Tuesday.

Members of Mrs May's own Conservative party led the revolt amid fears that delaying the decision raises the risks of Britain crashing out of the EU on 29 March with no deal at all.

It was the second setback in as many days for the prime minister, after MPs voted late yesterday to deny the government certain taxation powers in a no-deal scenario - another attempt to avoid such an outcome.

Speaking to MPs earlier, Mrs 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."

The parliamentary manoeuvres came at the start of five days of debate on the deal Mrs May struck with the EU last November, which has been strongly opposed from MPs on all sides of the House of Commons.

She set out further clarifications she hopes will win over her own Conservative MPs and the 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.

The backstop is designed to avoid a hard border on the island of Ireland post-Brexit

The British government has now offered guarantees to local Northern Ireland MPs over the operation of the backstop, and on the free flow of trade between Northern Ireland and mainland Britain.

However, DUP deputy leader Nigel Dodds said the proposals were "meaningless and cosmetic", adding he would keep pressing for changes to the overall deal.

DUP Brexit spokesperson Sammy Wilson told RTÉ's News at One programme that the paper was a piece of "window dressing".

He said it would ensure that neither the British government nor the Northern Ireland assembly would have any ability to stop the backstop being implemented.

Mrs May is also still seeking assurances on the operation of the backstop from European leaders, which she hopes to deliver before the vote next week.

The prime minister has already postponed the vote once -- in December -- to avoid defeat, and has said a loss for the government would plunge Britain into "uncharted territory".

But without the DUP's support, and with many of May's Conservative MP still strongly opposed, the deal cannot pass.

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Vardakar dismisses backstop veto proposal

Elsewhere, the Taoiseach has said the Government could not accept a situation in which the assembly held a veto over the "backstop".

Mr Varadkar said: "I don't think we can have a situation whereby the Northern Ireland executive or assembly has a veto power because that would essentially give one of the two communities veto power over the other." 

Speaking to reporters on a visit to Ethiopia, Mr Varadkar said that he had not yet had a chance to read the proposal, but would do so later.

He added: "When it comes to some of the things that are happening in London this proposal that parliament would be able to choose between the triggering of the backstop or an extension of the transition period, we're happy with that. It's something that had previously been discussed with the UK government.

"So essentially what that means is that after the two-year transition period towards the end of 2021 if at that point in time we don't have the new trade agreement in place the UK would be able to choose either an extension of the transition period or triggering the backstop. 

"We are happy with parliament making that decision because either way it means there will be no hard border and that's fundamentally the red line that we stand over."

Meanwhile, it has been confirmed that the Cabinet will next week discuss Brexit planning for ports, airports and medical supplies.

Although today's Cabinet meeting was chaired by Tánasite and Minister for Foreign Affairs Simon Coveney, the issue of Brexit was not on the agenda.

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In December, Mrs May postponed the crucial vote because it appeared her government would be defeated - something which would have plunged the Brexit process into further crisis.

While the current numbers suggest Mrs May will still lose the vote, her government has said it will go ahead next Tuesday evening.

British cabinet minister David Lidington has said that the remaining members of the EU will not unpick the Withdrawal Agreement in order to renegotiate a "magical alternative" deal.

Mrs May's de facto deputy told BBC Radio 4's Today programme: "I don't think that the British public are served by fantasies about magical alternative deals that are somehow going to sort of spring out of a cupboard in Brussels.

"This deal on the table has involved some very difficult give and take on both sides and if you go around and talk to the other EU 27 governments they will say that there are elements of this that cause them some political pain, but they are very clear, in conversations I have had with them, as well as their public statements, they ain't going to be going back and unpicking this for some brand new brilliant renegotiations.

"So, the choice that people have is this deal or it is no deal or it is, as some MPs advocate, to reverse the 2016 referendum entirely."

He said the government's Commons defeat last night was "inconvenient" and urged MPs who are against a no-deal Brexit: "You have to put in place an alternative - a deal to govern exit - and the deal that is available is the one the prime minister has negotiated over many months."

Twenty Conservative MPs joined opposition parties in backing a cross-party amendment to the Finance Bill intended to limit the government's powers to make tax changes in the event of no-deal.

Additional reporting PA