British Prime Minister Theresa May suffered a huge blow on Thursday as Dominic Raab quit as her Brexit secretary, saying he "must resign" over the proposed EU withdrawal agreement.

"I cannot reconcile the terms of the proposed deal with the promises we made to the country in our manifesto," he said in his resignation letter, published on his Twitter account.

"I believe that the regulatory regime proposed for Northern Ireland presents a very real threat to the integrity of the United Kingdom," he said.

Raab added that he was opposed to "an indefinite backstop arrangement" to guarantee the Irish border remains free-flowing, saying the EU would hold "a veto over our ability to exit".

"No democratic nation has ever signed up to be bound by such an extensive regime, imposed externally without any democratic control over the laws to be applied, nor the ability to decide to exit the arrangement," he wrote.

A junior minister has also quit May's government on Thursday over the proposed Brexit withdrawal agreement, saying it failed to leave Britain as a sovereign nation.

Shailesh Vara, a Northern Ireland minister, became the first member of the government to quit over the deal as May began trying to sell the draft accord to parliament.

The agreement "leaves the UK in a half-way house with no time limit on when we will finally be a sovereign nation", he wrote in his resignation statement, which he published on his Twitter account.

Vara backed Britain staying in the European Union in the 2016 referendum in which 52% of Britons opted to leave the bloc.He said the vote had to be delivered upon.

"With respect, prime minister, this agreement does not provide for the United Kingdom being a sovereign, independent country leaving the shackles of the EU, however it is worded," he wrote.

"We are a proud nation and it is a sad day when we are reduced to obeying rules made by other countries who have shown that they do not have our best interests at heart.

"We can and must do better than this. The people of the UK deserve better."

May won her cabinet's approval for the agreement during a five-hour meeting on Wednesday, an important step that helped allay growing fears of a disorderly divorce.

However, she faces a mutiny in her own party, which does not command a majority in parliament's lower House of Commons.

She will set out the terms of the agreement to the chamber, which must approve the deal before Brexit day on March 29.

European Council President Donald Tusk meanwhile called a leaders' summit later this month to seal the deal.

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