Guy Verhofstadt, the European Parliament's chief negotiator, says he does not accept any attempt to "bargain" between trade and security.

"I think the security of our citizens is far too important to start a trade off from one for the other," he says.

Jean-Claude Juncker has said he is "deeply sad" at the UK beginning the process of its departure from the EU.

Speaking in Malta, the European Commission president said the EU would formulate its response and negotiating approach at a summit next month but, when asked what his personal feelings were, he said he was saddened.

Asked whether he was concerned the UK was seeking a trade-off between trade and security, he said both factors "had to be taken into consideration". 

BBC Radio 5 live

Labour should be helping to get Britain "the best possible [Brexit] deal", but a deal is unlikely to be reached within two years, Labour MP Alan Johnson has told BBC Radio 5 live . 

The former Home Secretary said the “first issue” for the government had to be about assuring the future of EU nationals, and not using them “as a bargaining chip”.

“They’ve got an awful lot to do in two years – I don’t think I’m the only one who thinks that they’re not going to get this done. There’s everything from air traffic control, [to] the European Medicines Agency,” he said.

House of Lords
Parliament

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Crossbencher and Article 50 architect Lord Kerr of Kinlochard welcomes the "conciliatory" tone of Theresa May's letter.

However, he adds the government "will be judged by the words of those on the front bench", drawing contrasts between the words of ministers before and after the referendum, warning: "The country won't forget what it was told."  

Antonio Tajani, the president of the European Parliament, also refers to the question of joint security working between the UK and the EU.

He seems to respond to a line in Theresa May's Article 50 letter - that "cooperation in the fight against crime and terrorism" could be weakened if no deal is reached.

Some have seen this part as a veiled threat, although Downing Street denies this.

Mr Tajani says close cooperation on police matters should continue, "whether there is a deal or not".

Image source, Reuters

UKIP supporters have been out in force celebrating in both London and Brussels. As part of the festivities, an Article 50 cake has been consumed.

The president of the European Parliament, Antonio Tajani, is giving a press conference alongside the Parliament's negotiator, Guy Verhofstadt. Mr Tajani says it should be clear that being an EU member cannot be the same as not being a member, but refers to the "shared heritage" of the two sides and repeats Theresa May's line that the UK is leaving the EU, not Europe.

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Theresa May's predecessor has also been speaking today - David Cameron was addressing students in Ukraine.

The former PM defended his decision to call a Brexit referendum, saying the issue of the UK’s membership was “poisoning” UK politics, and that the “decisive” result of that referendum should be respected.

He said: “Britain always was uncertain and opposed to the idea of a deeper and more integrated union. We never liked the EU flag. We liked our own flag.”

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Parliament

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Lib Dem Lord Newby describes the activation of Article 50 as an "extremely sad day" but "sadness is a passive emotion", and his party does not intend to be passive. 

He says it remains his party's position that the people should decide whether to accept the final Brexit deal. 

He asks how leaving the EU can do anything other than diminish "liberal democratic values" mentioned by the prime minister. 

House of Lords
Parliament

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Shadow Lords leader Baroness Smith of Basildon responds to the statement for Labour. 

She tells peers that "there will be some who look forward to new opportunities, others will despair" but all are facing "uncertainty". 

To Jeremy Corbyn's six tests set out earlier, she adds her own, "the test of complete honesty" which she says she will be looking out for when the Brexit deal is eventually reached or not reached. 

Much has been said about the impact of the UK's EU departure on Northern Ireland. BBC experts have picked out the key issues to look out for as negotiations begin.

Article 50: What to look out for in Northern Ireland

Plaid Cymru's parliamentary leader Hywel Williams said the devolved administrations wanted more than consultation on powers to be "dribbled down".

Northern Ireland's two biggest political parties remain deeply divided on their views on Brexit.

Alex Forsyth
Political correspondent

The key points of Theresa May's letter dissected

5) Transitional arrangements

• What does the letter say? " We should work together to minimise disruption and give as much certainty as possible."
• What's the significance? Two years isn't a long time to rework a relationship that's lasted for more than four decades. There are swathes of regulation and legislation to unpick, discussions on a whole host of issues from immigration, trade, workers' rights and environmental regulation. The outcome will have an impact on businesses, investors and individuals. The prime minister again says she wants to avoid a "cliff edge" - a dramatic period of rapid change when the old rules cease to apply and the new ones start. Instead she suggests a "phased implementation", where the new way of working, once agreed, can be brought in gradually to "minimise disruption". Theresa May has talked of this before, but in the letter asks the EU to agree to this principle early on in talks. If it does, this effectively means the UK won't necessarily sever all ties with the EU when the negotiating period is up in two years' time. Instead there'll be a transition.

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One of the key figures in the Vote Leave campaign, Matthew Elliott, strikes an optimistic note, saying that having prepared in the nine months since the referendum the UK is "well-placed" to tackle the negotiations.

He also thinks trade negotiations will be more straightforward than some think, and says he detects a new sense of "self confidence" in the UK. "I think there's a slight spring in our step already," he says.

The World at One
BBC Radio 4

Leader of UKIP, Paul Nuttall, says there will be a need for compromise in the negotiations but that there are some red lines.

He told Martha Kearney that though "no one is talking about pulling up the drawbridge" on immigration, Britain needed to be able to control its own borders.

Alex Forsyth
Political correspondent

The key points of Theresa May's letter dissected.

4)The rights of EU citizens

• What does the letter say? "We should always put our citizens first."
• What's the significance? Many EU citizens living in the UK, as well as Britons living abroad, have expressed concern about the impact Brexit may have on their rights. Theresa May came under pressure to unilaterally agree to protect those EU citizens living in the UK - but repeatedly refused to do so, saying only it was important to get it resolved early. The EU refused to discuss it before Article 50 was triggered. Now that's happened, Mrs May makes clear an early agreement on this issue is still a priority. On this point the EU will agree - they too want this settled soon. But in today's letter, Mrs May acknowledges it is a complex area. Both sides may want it resolved, but the specifics are likely to be an early and tricky discussion.