THERESA May has begun the historic process of cutting Britain loose from the European Union, making clear there is now “no turning back”.

As a formal letter was handed over to Donald Tusk, the President of the European Council of leaders, in Brussels, the Prime Minister told MPs that she wanted Britain to have a “new, deep and special partnership” with the EU.

In a Commons statement, she stressed how by triggering Article 50 she was “giving effect to the democratic will of the UK people,” and described ending the UK’s 44-yearlong membership of the EU, and before it the EEC, as one of "the great turning points in our national story".

Mrs May insisted Brexit provided an opportunity to build a better, fairer and more unified Britain.

However, Jeremy Corbyn said the PM's Brexit plans were "reckless and damaging" as the Labour leader insisted his party would “not give this Government a free hand to use Brexit to attack rights, protections and cut services".

Tim Farron, the Liberal Democrat leader, accused the PM of “twisting the will of the people, leaping into the abyss without any idea of where our country will end up," and claimed it was not too late for voters to change their mind and reverse the Brexit decision in a second referendum.

Outwith Westminster, Nicola Sturgeon branded the triggering of Article 50 a “leap in the dark,” and described the Conservative Government’s approach as “hard-line” and a “reckless gamble”.

The First Minister, noting how the UK Government had only formally rejected the Scottish Government’s Europe paper on Wednesday afternoon as the Brexit process formally began, again argued that Scots must have the “final say on their own future once the terms of Brexit are clear”.

But UK ministers have made it clear there will be no talks on a second independence referendum until the Brexit process is over; which means another poll is unlikely to happen before 2020.

After being accused by Angus Robertson, the SNP leader at Westminster, of breaking her promise to get an agreement with the Scottish Government before triggering Article 50, Mrs May made clear: “We will negotiate as one United Kingdom, taking account of the specific interests of every nation and region of the UK.”

She explained that when powers were repatriated from Brussels, her Government would “consult fully” on which powers should reside at Westminster and which should be passed on to Edinburgh.

The PM stressed, however, that Holyrood and the other devolved parliaments would see a “significant increase in their decision-making power as a result of this process” but did not elaborate.

Declaring how the UK Government would use Brexit to “strengthen the Union of the four nations that comprise our United Kingdom,” she made an appeal for “unity” to MPs, saying this was a time for the country to come together.

But her comments in the Commons were heard over noisy expressions of protest from the SNP benches.

In a fiery exchange, Mr Robertson said: "The Prime Minister says she thinks Brexit will bring unity to the United Kingdom, it will not. On this issue it is not a United Kingdom and the Prime Minister needs to respect the differences across the nations of the United Kingdom.

"If she does not, if she remains intransigent, and if she denies Scotland a choice on our future, she will make Scottish independence inevitable."

Mrs May also faced accusations that she was threatening the EU with a withdrawal of Britain’s co-operation on security after her letter to Mr Tusk warned that a failure to reach agreement on trade would mean "our co-operation in the fight against crime and terrorism would be weakened".

MPs picked up on this with Mr Farron, describing it as "utterly scandalous", adding: "This letter is a blatant threat; security co-operation has been lumped together with trade."

Yvette Cooper, the former Shadow Home Secretary, insisted the PM should “not be trying to use this as a bargaining chip in the negotiations”.

The Labour MP, who now chairs the Commons Home Affairs Committee, added: "This is not a threat to the rest of Europe, it would be a serious act of self-harm. She should rule out now walking away with no security deal as our national security and public safety depend on it."

Later, however, No 10 denied Mrs May’s words in the Article 50 letter were a threat to the Brussels negotiators.

Her spokesman said: "The Prime Minister's words speak for themselves: it's a simple fact that if we leave the EU without a deal, then the arrangements which we have as part of our EU membership will lapse."

In her letter, Mrs May set out Britain's negotiating position, making clear she wanted a new trade deal to be negotiated at the same time as that for withdrawal; something the European Commission has ruled out.

Amid talk that Brussels was intent on presenting the UK with a £50 billion "divorce bill", the PM said she was ready to discuss a "fair settlement" of Britain's rights and obligations as a departing member but she gave no hint of the amount she was prepared to contemplate.

Mrs May also said she was ready to agree an implementation period following the formal date of Brexit in March 2019, to allow changes to business arrangements to be phased in gradually, avoiding a "cliff edge".

In the Belgian capital, a sombre-faced Mr Tusk held up the PM’s letter at a press conference as he sent a farewell message to the UK: "We already miss you. Thank you and goodbye."

The Council President confirmed he would set out his draft negotiating guidelines tomorrow, ahead of an extraordinary summit of the remaining 27 leaders on April 29, which is expected to pave the way for talks to begin in earnest in May.

Mr Tusk said the EU27 were "more determined and more united than before" with a strong mandate to defend the EU's interests in the "difficult" negotiations ahead.

But he said the invoking of Article 50 was not "a happy day" for him, for the EU or Britain, adding: "There is nothing to win in this process and I am talking about both sides. In essence, this is about damage control."