London  - Dozens of pro-Brexit lawmakers from British Prime Minister Theresa May's Conservative party have urged her to make a clean break with the European Union when Britain leaves the bloc in March 2019.

In a letter to May, the eurosceptics urged her to demand "full regulatory autonomy" for Britain after Brexit, sharpening divisions in the party ahead of a cabinet meeting on Thursday to discuss the differences.

A challenge to the Prime Minister

The 62 signatories from the Conservatives' European Research Group, which is led by right-winger Jacob Rees-Mogg, said Britain "must have the ability to change British laws and rules once we leave, rather than being a 'rule taker' without any substantive say in whatever Brussels decides."

It must be "free to start its own trade negotiations immediately" and sign new trade agreements during any transitional period, they said. Pro-EU Conservative lawmaker Anna Soubry said the letter was "a challenge to the PM's authority."

"The letter details the hard Brexit voters rejected in June. British business doesn't want it either," Soubry said on Twitter. May lost her parliamentary majority in a disastrous snap general election in June.

Many signatures "ideologically driven"

Another pro-EU Conservative, Nicky Morgan, told The Guardian that the letter amounted to a "ransom note." Soubry said May must "see off" the eurosceptic rebels, warning that she was "mistaken if she thinks she can appease them."

"Not all the signatures are ideologically driven, but many are – some would even abandon GFA to get their hard Brexit," she wrote, referring to the 1998 Good Friday Agreement, which underpins Northern Ireland's peace process.