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.

Earlier today Shailesh Vara, a junior minister, quit May's government over the proposed Brexit withdrawal agreement, saying it failed to leave Britain as a sovereign nation.

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 per cent 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."

Born in Uganda, Vara, 58, is a former vice-chairman of May's centre-right governing Conservative Party who has been a member of parliament since 2005.

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.