Theresa May is facing resistance from German Chancellor Angela Merkel and the European Parliament to her goal of conducting negotiations on Britain's future trade relations with Europe at the same time as talks on arrangements for Brexit.

Simultaneous divorce and trade talks are a key demand in the Prime Minister's letter to European Council president Donald Tusk triggering the two-year process of negotiating Brexit.

Mrs May told MPs that the delivery of the so-called Article 50 letter was "an historic moment from which there can be no turning back". It would implement the democratic will expressed by voters in last year's referendum by taking Britain out of the EU in "one of the great turning points in our national story", she said.

But she was accused of trying to make a trade-off between security and commerce, as the letter warned that failure to reach agreement on trade would mean "our co-operation in the fight against crime and terrorism would be weakened".

Home Secretary Amber Rudd later said the UK was "likely" to leave the EU's law enforcement agency Europol as part of Brexit, telling Sky News: "If we left Europol then we would take our information ... with us. The fact is the European partners want us to keep our information in there, because we keep other European countries safe as well."

Liberal Democrat leader Tim Farron described Mrs May's warning as "utterly scandalous", adding: "This letter is a blatant threat, security co-operation has been lumped together with trade."

And the European Parliament's Brexit co-ordinator Guy Verhofstadt said: "Security is far too important to start to bargain it against an economic agreement ... What we will never accept is that there is a trade-off between the one and the other."

But he shied away from suggestions that the PM's comments amounted to blackmail, telling reporters in Brussels: "I try to be a gentleman, so towards a lady I don't even use or think about the word 'blackmail'."

In her letter to Mr Tusk, Mrs May said it was "necessary to agree the terms of our future partnership alongside those of our withdrawal from the EU".

But Mrs Merkel gave a frosty reception to the PM's plea, saying that talks must first clarify how to unravel the commitments, rights and duties which the UK has entered into over its 44-year membership

"It is only if we have sorted that out that we can next - and I hope soon -talk about our future relationship," said the German Chancellor.

A draft resolution expected to be approved by the European Parliament on April 5 also stipulates that a trade deal cannot be sealed until after UK withdrawal. It proposes a three-year time limit for a transitional period to thrash out an agreement following the likely Brexit date in 2019.

Parliament president Antonio Tajani said that MEPs were ready to veto any deal that harmed the rights of EU citizens living in the UK. And he warned that exiting without a deal would be a "catastrophe" for Britain, leading to cuts in trade, higher food prices and massive disruption to traffic at Dover.

The six-page Article 50 letter, signed in ink by the Prime Minister, was hand-delivered to Mr Tusk in his Brussels office by UK permanent representative Sir Tim Barrow at around 12.20pm, just minutes before Mrs May made a statement to MPs on her plans.

Mrs May said the UK wanted a "deep and special partnership" with the future EU of 27, but restated her determination that Britain should regain control of its borders, leave the single market and escape the jurisdiction of the European Court of Justice.

Amid expectations of a £50 billion "divorce bill" from the European Commission, Mrs May said she was ready to discuss a "fair settlement" of Britain's obligations, but gave no hint of the amounts she is prepared to contemplate.

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

A sombre-faced Mr Tusk held up the letter at a Brussels press conference as he sent a farewell message to the UK: "We already miss you. Thank you and goodbye."

The European Council president confirmed he would set out his draft negotiating guidelines on Friday, 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.

The EU 27 were "more determined and more united than before", with a strong mandate to defend the EU's interests in the "difficult" negotiations ahead, he said.

But he said Article 50 was not "a happy day", adding: "There is nothing to win in this process - and I am talking about both sides. In essence, this is about damage control."

Labour leader Jeremy Corbyn denounced Mrs May's plans as "both reckless and damaging" as he warned his party "will not give this Government a free hand to use Brexit to attack rights, protections and cut services."

Mr Farron said it was not too late for voters to reverse the Brexit decision in a second referendum.

"The Prime Minister is twisting the will of the people, leaping into the abyss without any idea of where our country will end up," said the Liberal Democrat leader.

"Democracy didn't end on 23rd of June - and it hasn't ended today either. The people can have their say over what comes next."

European Commission president Jean-Claude Juncker said the UK's decision to quit the bloc was a "choice they will regret one day".

Speaking in Valletta, Malta, he also left the door open for an "associative" citizenship of the EU for Britons.

He said: "It's not my working assumption that this will happen but I know mainly the chief negotiator of the European Parliament is asking for this.

"It would not disturb me if this happened. But this cannot be the answer. It doesn't take away from Brexit all the dangers and problems. It would be an answer for citizens."

Malta's PM Joseph Muscat - who holds the six-month rotating presidency of the European Council - said it was a "bitter, bitter" day.

"There's nothing sweet about what's happening today," he added.

Mrs May refused to commit to a significant reduction in immigration once Britain has left the bloc.

She told the BBC's Andrew Neil there were "so many variables" that influenced the number of people moving to the UK but insisted "we will see a difference in the number of people coming in".

The PM said she wants "everybody to know" what the withdrawal arrangements are and the terms of the future relationship by the end of March 2019 but there "may be a period of implementation" after the UK leaves the EU.

Asked if free movement would end within two years, Mrs May replied: "We want to have the agreements done in two years. There may then be a period when we are implementing those arrangements."

During the half-hour prime time interview on BBC One, the PM sidestepped questions over whether Britain would be left with a divorce bill.

"There isn't a formal demand. The negotiations haven't started yet. I'm very clear about what the people here in the UK expect but I'm also clear that we are a law abiding nation, we will meet obligations that we have."

Mrs May added: "I did campaign for Remain and I did vote to Remain but I also said I didn't think the sky would fall in if we left the European Union and it hasn't."