FILE PHOTO: Britain's Prime Minister Theresa May leaves 10 Downing Street in London, Britain, May 23, 2018. REUTERS/Toby Melville/File Photo

LONDON (Reuters) - More than 30 Conservative lawmakers have signed a letter demanding Theresa May “get tough” in negotiations to leave the European Union, increasing the pressure on the British prime minister before a crunch government meeting.

May has pledged to overcome deep divisions in her cabinet of ministers at a meeting at her country residence this week and come up with a blueprint for the future relationship with the EU, a step needed to move forward all-but-stalled Brexit talks.

The letter from Conservative lawmakers adds to the pressure on the prime minister, who has so far been reluctant to spell out detailed Brexit plans because of the rifts inside her own cabinet over the terms of Britain’s biggest foreign policy shift for decades.

Dated June 29, the letter calls on May to “demonstrate courage and leadership in the face of those who seek to undermine the express wish of the British people in the 2016 referendum”.

“Our departure must be absolute. We must not remain entangled with the EU’s institutions if this restricts our ability to exercise our sovereignty as an independent nation. Anything less will be a weakening of our democracy. Britain must stand firm.”

It also lists what the lawmakers will not accept, including any extension of a transition period beyond Dec. 31, 2020, remaining part of a customs union and any deal which fails to link the payment of a divorce bill to getting a good trade deal.

With only nine months to go before Britain leaves the EU, May is also under pressure in Brussels to set out her vision for future ties and offer ways of meeting a promise to prevent a return to a hard border on the island of Ireland.

The lack of a proposal on how to keep a near-invisible border between the British province of Northern Ireland and EU member Ireland has stalled Brexit talks, increasing frustration among the other EU leaders.

Last week, they again offered more flexibility if May would rethink her so-called red lines, including a commitment to leave the bloc’s customs union, a pledge some EU officials say complicates any potential solution to the border question.