Former British Foreign Secretary Boris Johnson was the first big-name politician to say on Tuesday night that he would vote against a so-called Brexit agreement brokered between Brussels and the British government.

Prime Minister Theresa May spent Tuesday night meeting her top ministers one by one at 10 Downing Street to discuss details of a draft agreement for Brexit. It was worked out with negotiators from both sides aimed at paving the way for Britain's exit from the bloc next March. Each minister was allowed to read the draft agreement.

May has called an emergency cabinet meeting Wednesday afternoon to get approval from her top ministers for what would be an historic agreement.

The pound sterling surged on the currency market following news of the breakthrough, but financial observers said the rally could be short lived depending on how the cabinet votes on Wednesday.

Within minutes of details being announced about the breakthrough, questions and doubts were being raised both in the British mainland and across the Irish Sea in Northern Ireland.

Boris Johnson said he would vote against the deal, claiming it would transform Britain into a vassal state of Europe, and be the biggest change in the UK for a thousand years.

"For the first time in a thousand years, this (British) parliament will not have a say in the laws of this country, something which would be utterly unacceptable. I hope the cabinet will do the right thing and I hope they will chuck it out," Johnson said in a media interview.

Conservative MP and influential Brexiteer, Jacob Rees-Mogg described the draft agreement as a failure of the government's negotiating position and a failure to deliver on Brexit. He added that the deal, as reported, would keep Britain in the European Customs Union and "de facto" in its single market.

More troubling for May was a comment made by Northern Ireland politician Nigel Dodds of the Democratic Unionist Party. His party shores up May's minority government in the House of Commons.

"We have to see the details, but it appears to be a UK-wide customs agreement but deeper implications for Northern Ireland both on customs and single market. If that means that we're taking the rules and laws set in Brussels, not in Westminster or Belfast, then that's unacceptable," he said on Tuesday night.

The document, said to run to 500 pages, outlines how one of the trickiest issues of the negotiations, the land border between Northern Ireland and the Republic of Ireland, would be resolved.

It raised even more questions about what would happen if the deal is rejected when it is put before the British Parliament before the end of the year.

Many of Britain's leading politicians and political commentators, as well as constitutional experts are waiting to read the small-print of the draft document before passing judgement.

Few commentators predicted it would spark a snap general election on the basis the last election called by May backfired when she lost an overall majority in the House of Commons.

The big fear is that another election now could see the main opposition Labor Party winning, putting left-winger Jeremy Corbyn in No.10.

Labor's chief Brexit spokesman Keir Starmer said: "Given the shambolic handling of these negotiations, this is unlikely to be a good deal for the country. It's not in the national interest to vote for a bad deal."

Labor Leader Jeremy Corbyn said "We will look at the details of what has been agreed when they are available, but from what we know of the shambolic handling of these negotiations, this is unlikely to be a good deal for the country."

The leader of the minority Liberal Democrat Party, Vince Cable, who backs a new referendum, said: "Before the ink is dry, the Conservative Party will tear into what little Theresa May has been able to agree. "The prime minister now faces a defeat in Parliament, as a majority will be hard or impossible to secure for what she has come up with."

In London on Tuesday night Mayor Sadiq Khan said Cabinet ministers must put aside their personal interests and ambitions and do what's in the national interest.

"It's clearer than ever that the British public must now be given a say on the terms of the final deal, with staying in the EU an option," Khan said.

In Dublin Irish Prime Minister Leo Varadkar told his ministers to be on "standby" for a special cabinet meeting Wednesday to update them on Brexit.

The next 24 hours will prove pivotal in the long process of Britain's exit strategy after 45 years of EU membership.