Prime Minister Theresa May is facing a potentially dangerous outcry from 62 members of her own party who are demanding a quick, clean break from the European Union, just as she tries to finalize her Brexit plans.

The group of Conservative lawmakers challenged May to take a harder approach on two key issues: how far British rules should move away from those of the EU after Brexit, and the nature of the transition period that businesses desperately want.

While insisting they back May’s leadership, the Tories link their support to her delivering the kind of decisive split that they want Brexit to be. They number more than enough to trigger a leadership bid against May, whose government lacks a parliamentary majority.

“We are writing to reassure you of our continued, strong backing for the clear vision of an internationally-engaged, free-trading, global Britain which you laid out,” said the letter, before offering “some suggestions” on how to achieve it. “Your government must have the ability to change British laws and rules once we leave, rather than being a ‘rule-taker,”’ it said.

The letter, organised by the European Research Group of euroskeptic Tories, is a potentially destabilizing development at a highly sensitive time for May’s Brexit preparations. She was left weakened by last year’s disastrous election.

May has faced criticism from colleagues who say she lacks vision, while Brexit supporters fear she’ll sell them short and tie Britain too closely to EU rules.

Her government will publish a written statement Wednesday setting out its response to the European Union’s negotiating guidelines on the transition period. Ministers have already said that they want protections for the U.K. from EU regulations introduced during that time.

Jacob Rees-Mogg, the chairman of the European Research Group, met May with some of his colleagues in her 10 Downing Street office on Tuesday to discuss their demands. May’s office declined to comment on the letter or the meeting, though one official said May welcomed all contributions to the Brexit debate.

The lawmakers’ letter was dated Feb. 16.

After months of internal divisions, May is aiming to win support on Thursday from her inner circle of cabinet ministers for her blueprint for the post-Brexit trade deal between the U.K. and the EU. She will then set out her detailed plans in a speech likely to come next week, before formal trade talks begin in March.

While May hasn’t yet formalized her plans, the broad shape of the deal she wants is becoming clear: The U.K. will stay closely aligned to EU rules in many areas, including manufactured goods, while breaking away immediately in others, such as agriculture and financial services.

Brexit Secretary David Davis proposed that both sides could mutually recognize each other’s regulations as acceptable in future, removing the need for cumbersome negotiations on developing shared regulatory regimes. Euroskeptics see a risk this would lead to the U.K. following EU rules.

The letter to May calls for “full regulatory autonomy” post-Brexit and makes clear that she will have to fight them if she’s thinking of allowing EU regulators to take a role in British affairs, for example in areas such as data sharing.

It also states that the transitional phase—or implementation period—that’s currently being negotiated must meet their own criteria and be based on World Trade Organization principles.

The Times newspaper reported that this would mean the trade deal has to be completed before the transition starts, whereas the EU envisages talks continuing during the grace period. Rees-Mogg has said the government’s plans for the transition will render the U.K. a “vassal state” of the EU.

While the letter stopped short of telling May to drop her transition plan altogether, it does not endorse the “no change” period that May and her ministers have told businesses they can expect next year.

The Tories are also clear that Britain “must be free to start its own trade negotiations immediately,” an ambition May says she shares.