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Theresa May's Brexit deal has been dealt a fresh blow after being given a highly critical verdict from MPs.

The Commons Committee on Exiting the European Union said it doesn't give enough clarity or certainty about the future, describing it as a "huge step into the unknown".

The cross-party report comes days before the Commons is due to vote on the proposed deal on Tuesday.

Meanwhile Labour said the prime minister's plan is "dead in the water".

'Significant uncertainty'

The committee described the draft political declaration on the future relationship between the EU and the UK as "neither detailed nor substantive".

Its report said that meant there was still "significant uncertainty" about the terms of trade after the transition period elapses at the end of December 2020 - and that "the extent of our access to EU markets will depend on the degree to which we adhere to its rules".

"After 20 months of negotiations, we only know the terms of the UK's departure but not the nature of the future relationship with the EU," the committee said. "The prime minister's deal fails to offer sufficient clarity or certainty about the future."

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The MPs said the government had failed to set out objectives that are "realistic, workable and have parliamentary support". They said this meant negotiations with the EU on the future relationship with the UK would be more complicated and could take "significantly" longer.

On the issue of the Irish border, the report said there were no "realistic, long-term proposals" on how to keep it open while leaving the single market and customs union.

The backstop is meant to be a last resort to maintain an open border on the island of Ireland, in the event that the UK leaves the EU without securing an all-encompassing deal. It would keep the UK under EU customs rules in order to prevent a hard border.

But the report says this would "result in immediate barriers to UK-EU trade in goods and services".

It also said a Canada-style free trade deal with the EU would not be a viable option as it would "not ensure the type of friction-free trade with the EU that many UK companies with just-in-time supply chains need."

Image copyright Getty Images Image caption Hilary Benn said the government had "refused to face up to hard choices"

Hilary Benn, chair of the committee, said the government had "refused to face up to the hard choices confronting us".

"The political declaration falls far short of the 'detailed and substantive' document promised by former secretaries of state and by the EU chief negotiator, Michel Barnier," he said.

"It does not give the British people or our businesses the clarity and the certainty they need about our future trading relationship with the EU in five or 10 years' time."

He said he hoped the report would help other MPs when making their decision ahead of the vote next week.

Shadow chancellor John McDonnell said Labour's alternative Brexit plan could prevent a political vacuum engulfing Westminster if Mrs May's deal is defeated.

He said his party's proposals could "give a level of certainty" for the next stage of the Brexit negotiations.