The key European Union Brexit negotiator with the UK has said his team is "ready".
Michel Barnier also tweeted a photograph of his team who he said would "work for the 27 European Union members" during Brexit negotiations.
ITV News' Security Editor Rohit Kachroo said the letter from Theresa May triggering Brexit negotiations suggests the UK will focus on security as a negotiating tactic.
"Alongside economy, frequent mentions of security and terror in PM's article 50 letter," he said. "Clearly, UK will use its security clout as leverage."
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The 27 other members of the European Union have vowed to "act as one and preserve its interests" in Brexit talks with the UK.
A statement issued by the European Council said the talks would start by focusing on arrangements for an "orderly withdrawal" from the bloc.
The council, pledged to approach the talks constructively and hoped the UK would be a "close partner" after Brexit.
"We regret that the United Kingdom will leave the European Union, but we are ready for the process that we now will have to follow.
"Our first priority will be to minimise the uncertainty caused by the decision of the United Kingdom for our citizens, businesses and member states."
European Council President Donald Tusk has received the letter from Theresa May triggering Article 50.
At a press conference in Brussels he said "there is no reason to pretend that this is a happy day, either in Brussels or London".
He added: "After all, most Europeans including almost half the British voters wish that we would stay together not drift apart."
Mr Tusk went on to say "as for me I will not pretend I am happy today".
"There is nothing to win in this process and I am talking about both sides. In essence, this is about damage control," said Mr Tusk.
The European Council President said the EU has been strengthened by the UK leaving as "we are more determined, more united than before".
He added a final message to Britain: "We already miss you, thank you and goodbye".
Jeremy Corbyn has said Labour will ensure the Government is held to account at every stage of the Brexit.
He told the Commons Theresa May is "threatening to take this country in is both reckless and damaging".
He added he would not give the prime minister a "free hand" to use Brexit to attack rights and cut services.
The contents of the letter to the European Union signaling Britain's intent to leave the EU have been revealed.Read the full story ›
A German government spokesperson has said that Britain will remain "an important partner in NATO and as a European partner" after Brexit.
As the Prime Minister triggers Article 50 taking Britain out of the EU Germany added that both the UK and the EU have an interest in "keeping good relations".
The Germany Foreign Ministry spokesperson however criticised the two-year timetable as "damn narrow".
Issues that need clarification include the right of EU citizens in Britain, said the spokesperson adding that "uncertainties are bad for Britain and the EU".
"We are watching carefully constitutional developments in Britain, including possible Scottish Independence Referendum", they added.
Theresa May told MPs she believes Britain's "best days lie ahead".
The prime minister said she would work for the "best possible deal" for Britain after Article 50 was formally triggered.
She added: "Now is the time for us to come together to be united across this House and across this country."
Theresa May told the Commons: "This is an historic moment from which there can be no turning back".
She made the announcement as Article 50 was formally invoked when the letter Mrs May had signed was received by EU Council President Donald Tusk.