Image copyright Reuters Image caption The prime minister says the UK wants to remain friends and partners with the EU

Theresa May has said the UK is facing "one of the most significant moments" in its recent history as she prepares to begin the process of leaving the EU.

The prime minister, who will officially tell the EU of the UK's desire to leave on Wednesday, said her goal was a "deep and special partnership" after Brexit.

A "global Britain" could build new alliances outside the EU, she added.

But a group of pro-Remain MPs said she would struggle to meet her goals and must be held accountable if she fails.

Ahead of formally triggering Brexit using Article 50 of the Lisbon Treaty, Mrs May spoke on the phone to EU Council president Donald Tusk, European Commission president Jean-Claude Juncker and German Chancellor Angela Merkel.

On Wednesday, the prime minister will officially tell the EU's other 27 members that the UK wants to pull out, just over nine months after the British public backed withdrawal by a margin of 51.9% to 48.1% in a referendum.

By triggering Article 50, Mrs May will set in motion a two-year process in which the terms of exit will be negotiated. Unless both sides agree to extend the deadline for talks, the UK will leave on 29 March 2019.

Get news from the BBC in your inbox, each weekday morning

The two sides will also try to agree the basis of the UK's future relationship with the EU although some experts, including the former top civil servant at the Foreign Office, have said this could take many more years.

Speaking at a Qatari investment forum in Birmingham - where the Gulf State announced £5bn of further investment in the UK - Mrs May said this was "one of the most significant moments the UK has faced for many years".

"Tomorrow we begin the negotiations to secure a new deep and special partnership with the European Union. As we do so I am determined we should also seize this historic opportunity to get out in to the world and to shape an even bigger role for a global Britain.

"This means not just building new alliances but going even further in working with old friends who have stood alongside us for centuries."

A group of MPs who all backed the Remain campaign in last year's poll and are now part of the Open Britain group said the "phoney war" was coming to an end and voters must be able to hold Mrs May to account over whether the UK emerged stronger and more prosperous outside the EU.

Although the group, including former Lib Dem leader Nick Clegg, ex Conservative education secretary Nicky Morgan and Labour's ex-shadow chancellor Chris Leslie, said they wanted the best outcome for the country, they feared this was unlikely as the PM's approach was fraught with contradictions.

Image copyright AFP Image caption Vote Leave campaigners made a series of fiercely-contested claims during the referendum

The PM has said the UK will leave the single market but wants the greatest possible access to it and while leaving the customs union as it stands, she wants a similar arrangement that provides "frictionless" trade across borders.

Mrs May, they said, should be judged not only on the promises her government had made in recent months but on the "expectations" they said people had when they voted Leave last year, including that Brexit would lead to a fall in migration and free up extra funding for the NHS and other domestic services.

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

"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 UK will be protected".

They added: "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."

But Mr Duncan Smith, the former work and pensions secretary and leading Brexit supporter, said that the EU had "decided to leave the UK" in the late 1980s when it embarked on what he said was a one-way process of economic and political union.

Writing for the ConservativeHome website, Mr Duncan Smith suggested that, from that moment on, the UK's exit had been largely inevitable and he was confident about what lay ahead.

"We do so with political leaders in the EU beginning to use common sense terms as they now speak of needing good arrangements with the UK to protect their markets and their access to financial services.

"After all, it must be in everyone's interest, as European Commission President Juncker said the other day, for the UK and EU to part as friends, co-operating and trading."

On a visit to Brussels, London Mayor Sadiq Khan said the EU should enter the talks with confidence and would be making a mistake if it sought to punish the UK for deciding to leave.

"I say this with friendship and all due respect," the Labour politician said. "But a bad Brexit deal that hurts London would hurt the EU too...There is no need - as some have suggested - for the EU to send a message or to instil fear by punishing the UK.

"Because a proud, optimistic and confident institution does not secure its future by fear."