Pro-Leave Conservative MPs describe Theresa May's Brexit plan as the "worst of all worlds" in an 18-page letter.

On Friday, the prime minister persuaded her ministers to support a Brexit plan which would keep the UK wedded to EU's standards and make new free trade deals with certain countries unlikely.

The proposal, regarded as a soft form of Brexit, is designed to preserve frictionless trade and the Irish border.

However, Brexiteers in the Conservative party say it would make the UK a "vassal state."

LONDON — Pro-Brexit Conservative MPs have put together an 18-page critique of Theresa May's Brexit plan, in which the prime minister's proposal for a soft exit is described as a "worst of all words 'Black Hole Brexit'."

Prime Minister May persuaded her Cabinet to back a proposed future relationship with the European Union after a day-long meeting at Chequers, the prime minister's official retreat, on Friday.

The proposal has widely been interpreted as a distinctly soft form of Brexit, and much softer than the sort of Brexit she promised in speeches earlier in her leadership.

Under May's proposal, the UK will remain fully aligned with the EU on the trade of goods and collect EU customs tariffs on the bloc's behalf in order to preserve frictionless trade, including across the Irish border.

The UK Parliament would be able to refuse to apply new EU rules relating to the trade of goods. However, crucially, the UK would face adverse consequences for doing so, like reduced access to the single market.

By remaining wedded to EU standards, it would be extremely difficult for the UK to strike new free trade deals with countries with lower standards on certain products, like the United States of America, for example.

The letter lambastes the prime minister's plan, claiming it would leave the UK:

"stuck permanently as a vassal state in the EU's legal and regulatory tarpit;

"still as to obey EU laws and ECJ rulings across vast areas, cannot develop an effective trade policy or adapt our economy to take advantage of the freedom of Brexit;

"and has lost its vote and treaty vetoes rights as an EU Member State."

It is not clear how many Conservative MPs support the letter.

However, at least 10 are expected this weekend to submit letters calling for May to be replaced as leader, and more could follow if the prime minister doesn't satisfy the pro-Brexit wing of her party in her response to parliamentary questions about her plan next week.

At the Chequers get together where ministers agreed to support the plan, Foreign Secretary Boris Johnson reportedly described it as a "turd" before backing down and supporting the prime minister.

His fellow Cabinet colleague and Brexiteer, Andrea Leadsom, is thought to have told May that she "hated" the plan but would give the prime minister her support.

Theresa May addresses members of her cabinet at an all-day Brexit session in Chequers, the Prime Minister's country retreat in Buckinghamshire. UK Govt handout, via Getty Images

A Survation poll released on Saturday night found that 42% of Brits regarded May's proposed Brexit as the "wrong deal for Britain" while 26% said it was the "right deal" for the country.

All eyes are on now Brussels, where the EU will decide how to respond to the plan.

As Business Insider reported last week, it is expected to reject the plan in its current form, as it does not want the UK to have full single market access for goods alone. There is a feeling in Brussels that May's shift to a softer Brexit suggests she'd go further and accept the softest form — staying in the single market and customs union.