A cross-party group of leading MPs hope to hold the UK government to account by publishing a so-called "Brexit Contract" on Tuesday (28 March). The document sets out what Theresa May and her ministers have promised to deliver from the UK's split from the EU.

Former Conservative Education Secretary Nicky Morgan, Labour MP Chris Leslie and ex-Deputy Prime Minister Nick Clegg will unveil the contract at an event in central London just a day before May invokes Article 50 of the Lisbon Treaty and triggers two-year-long Brexit talks.

"The phoney war is over," the Open Britain campaign supporters said.

"There is no mandate for the form Brexit takes: responsibility for the outcome now rests with those conducting negotiations and those advocating a hard Brexit.

"As this process takes place we must ensure people are equipped to make judgements on whether negotiations meet the expectations they had when they voted last year, whether promises made then and since are being fulfilled, and whether our country and economy are likely to be stronger as a consequence of decisions now being taken.

"A clear direction of travel has been set by the government – and it is largely based on that set by the Vote Leave campaign.

"Vote Leave and the Government have made specific promises: leaving is a cost-free option; trade will be enhanced not hampered; there will be major savings from the EU budget; core arrangements with the EU, for example over national security, will remain unchanged; and the integrity of the United Kingdom will be protected."

May, speaking at Lancaster House in London in January, said the government would not seek to maintain the UK's full access to the EU's single-market.

Sir Tim Barrow, the UK's chief representative to the EU, and Brexit Secretary David Davis are expected to lead negotiations with the EU. French politician Michel Barnier is leading the EU Commission's task force for the talks.

"I want us to be a truly Global Britain – the best friend and neighbour to our European partners, but a country that reaches beyond the borders of Europe too," May said. "A country that goes out into the world to build relationships with old friends and new allies alike.

"I want Britain to be what we have the potential, talent and ambition to be. A great, global trading nation that is respected around the world and strong, confident and united at home."