London - Britain's former foreign secretary Boris Johnson renewed his attack on Prime Minister Theresa May's Brexit proposals on Thursday, urging her to drop her plan for a free trade area with the EU that she agreed with her cabinet.

Pro-Brexit Johnson, who said he resigned because he opposed the plan, slammed the proposals, which were hammered out during a cabinet meeting on Britain's exit from the European Union earlier this month at Chequers, the prime minister's retreat outside London.

Britain to leave the single market for services

"You can't leave an organization and still be bound by its rules. But that is what the Chequers white paper means," Johnson wrote in the right-of-centre Spectator magazine, which he once edited.

"It is vassalage, satrapy, colony status for the UK.

"For the first time in a thousand years our laws will be made overseas, enforced by a foreign court," said Johnson, widely considered to be May's biggest potential rival if she faces a leadership challenge.

"It can’t and won’t work," he added, urging May to "chuck Chequers."

Following the Chequers meeting, May set out a plan involving the creation of a free trade area for goods and agricultural products between Britain and the EU, in effect keeping the sector under EU single market rules and ensuring an open border between Northern Ireland and Ireland.

But Britain would leave the single market for services, which accounts for some 80 per cent of Britain's economy.

After announcing the Chequers agreement, May was rocked by the resignation of Johnson, former Brexit secretary David Davis and a high-profile junior Brexit minister.

Britain's Brexit Secretary Dominic Raab headed to the latest round of talks with EU Brexit negotiator Michel Barnier in Brussels on Thursday, pledging to honour the wishes of readers of populist tabloid The Daily Mail for him to "get on with it."

"It has been more than two years since the referendum, and I know many people want us to get on and deliver on the verdict of the British people," Raab wrote in the newspaper.

"Taking back control of our money, our law, our borders - and our country's future," he said.

"We are striving for the best deal"

May's proposals set out "a principled and pragmatic Brexit," Raab said, insisting that it would leave Britain "outside of the political institutions in Brussels that so many of us campaigned to leave."

He said 80 per cent of Britain's withdrawal deal had been agreed, adding that he and Barnier would discuss "how we complete the remaining 20 per cent, including guarantees to avoid a hard border between Ireland and Northern Ireland."

"We are striving for the best deal," Raab wrote. "But in case our ambition and energy are not matched, we are stepping up our preparation for no deal."

Speaking at an agricultural show on Thursday, May said wanted to cretae a new system of farming subsidies and "make the most of the freedoms provided by Brexit to design a new scheme that is less bureaucratic."

In Brussels later Thursday, Barnier said that British commitments regarding human rights and the role of the European Court of Justice made in the white paper enlarge the extent to which Britain and the EU will be able to cooperate on security issues.

However, he rejected British proposals to collect customs duties on behalf of the EU, put forward as part of efforts to ensure smooth transit at frontiers between Britain and the EU and maintain an open border between the Republic of Ireland and Northern Ireland.

"The EU cannot, and the EU will not, delegate the application of its customs policy and rules, VAT and excise duty collection to a non-member who would not be subject to the EU's governance structures," Barnier said. Just like Britain, the EU also wants to maintain control of its money, laws and borders, Barnier noted.