No hard border among May's key principles for final Brexit deal

And she included a commitment to no hard border between Northern Ireland and the Republic in her seven guiding principles for the negotiations.

In her letter to European Council President Donald Tusk triggering the start of the formal Article 50 withdrawal process, the prime minister said she was seeking a “deep and special partnership” between Britain and the EU after Brexit.

While she recognised there had to be a “fair settlement” of the UK’s outstanding obligations, she made clear she did not accept the demand of the EU’s chief negotiator Michel Barnier that this had to be agreed before trade talks could start.

Ministers have previously indicated that any financial settlement would be far lower than the £50 billion that Mr Barnier is reportedly seeking.

Mrs May also issued a direct warning that failure to reach an overall agreement would damage security co-operation, weakening their joint efforts to combat crime and terrorism.

She set out seven key principles that she said should shape the forthcoming talks.

• The two sides should engage “constructively and respectfully” in a spirit of “sincere co-operation”. The UK was not seeking membership of the single market as it accepted there could be no “cherry-picking” when it came to principles behind it.

• The interests of citizens should come first, with the aim of securing an early agreement on the rights of EU nationals living in UK and British nationals in the EU.

• There should be a “comprehensive agreement”. It should include a “fair settlement” of the UK’s rights and obligations as a departing member of the EU to be negotiated alongside a wider agreement on the terms of Britain’s future “partnership” with the EU.

• There should be a minimum of disruption. In order to avoid any “cliff edge” when the UK leaves the EU, there should be “periods of implementation” to enable the adjustment to the new arrangements to take place in a “smooth and orderly” way.

• The UK’s “unique” relationship with the Republic of Ireland and the Northern Ireland peace process should be protected, with no return to a “hard border” between the North and the South.

• Technical talks on detailed policy areas should begin as soon as possible, while seeking a free trade agreement of “greater scope and ambition than any such agreement before”.

• The two sides should work to advance and protect their “shared European values”, ensuring Europe remains strong and prosperous while maintaining its ability to defend itself from security threats.