Prime Minister Theresa May said it was her "expectation" that the Brexit process would result in a "significant increase" in power for the devolved administrations.

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Norman Smith
Assistant political editor

In response to criticism of Theresa May's comments about how security co-operation may be affected in the absence of a broader Brexit deal, Downing Street has been quick to insist that she was not issuing a threat. 

At a No 10 briefing, reporters were told it was a simple statement of fact that existing EU security arrangements would lapse in such a scenario. 

It is not clear what this would involve but it is thought to include the European Arrest Warrant, Europol but not intelligence sharing nor Nato co-operation.

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Sir Gerald Howarth says the prime minister has demonstrated "outstanding leadership". 

The Conservative MP says he campaigned himself in 1975 to leave the Common Market and salutes her "determination" in securing the very best deal for the UK and "our European partners".

Ms May says it is right that we should "come together now" to get the best possible deal.

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Labour MP Jonathan Reynolds asks the prime minister to confirm what will need to happen in order to ratify a negotiated deal with the EU. 

Will it will require ratification by "every single other" EU state and their national parliaments? he asks. 

Ms May tells Mr Reynolds that, overall, it "will be necessary" for the European Parliament and its nation states to ratify the deal. 

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This is a great day for our country, says Conservative Mark Francois.

He adds that it is in Britain's DNA to be a free trade, outward looking nation.

The prime minister agrees that the UK's outward looking spirit "will stand us in good stead".

Alex Forsyth
Political correspondent

The key points from Theresa May's letter dissected

2) What if we don't reach a deal?

• What does the letter say?  "The UK wants to agree with the EU a deep and special partnership."
• What's the significance? When Theresa May gave her first in-depth insight into her vision for Brexit in a speech at Lancaster House earlier this year, she said "no deal for Britain was better than a bad deal". That led to much talk about what "no deal" would mean - for both the EU and the UK. In today's letter, the prime minister didn't explicitly repeat what she said previously. Instead, she reiterated that the UK wants to agree a "deep and special partnership with the EU" and failure to do so would weaken co-operation in the fight against crime and terrorism. Rather than threatening to walk away, she is attempting to highlight the mutual benefits to both the UK and the EU of reaching an agreement about their future relationship and warning of the dangers if that doesn't happen. It's a carrot rather than a stick, albeit one that comes with a health warning.

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Alex Forsyth
Political correspondent

1) The timing of the talks: what's discussed when?

• What does the letter say? "We believe it's necessary to agree the terms of our future partnership alongside those of our withdrawal from EU."

• What's the significance? This is all about the timing of the talks and is likely to be a key point of contention.The EU Commission's chief negotiator Michel Barnier has made his view clear - the UK and the EU should agree the terms of the UK's withdrawal before negotiating any future trade deal. He's said "putting things in the right order" maximises the chances of reaching an agreement. However the UK takes a different view - and one Theresa May chose to reiterate several times. She says the divorce and the new relationship should be discussed at the same time. This is the shaping up to be the basis of an early row if both sides stick to their positions.

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Immigration is likely to be a big factor in the upcoming talks. Here is what the BBC's home editor Mark Easton thinks are the main issues at stake.

The prime minister sees controlling immigration as a red line in the Article 50 negotiations, but whether the end of free movement will see a sharp fall in European migration to the UK once it leaves the EU is much less clear.    

Ministers recognise an immigration "cliff-edge" would be problematic for social care, health, construction, hospitality and agriculture, sectors, that currently rely on significant numbers of EU migrant workers.

It is suggested the UK might negotiate transitional arrangements lasting several years after Brexit.

How and for how long such arrangements might operate will be a key question in the Article 50 talks.

The government has said it will work with UK businesses to devise new immigration rules that allow them "to continue to thrive".

This is likely to mean a visa system, similar to the one already used for non-EU migrants. Read more

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The CBI is urging the prime minister to pursue some "early wins" during the negotiations, including a guarantee of the rights of EU residents to remain in the UK and assurances that firms won't face a "cliff edge" in changes to regulations and possible new tariff barriers.

The business group's president Paul Drechsler said the UK must enter the negotiations in an "ambitious and level-headed" frame of mind. 

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Unions have urged Theresa May to take all the time she needs to get the right deal for British workers.

TUC General Secretary Frances O'Grady said hard-won employment protections must not be sacrificed and British workers should not be any less secure than their French, German or Dutch counterparts.

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Other European governments are starting to react to the triggering of Article 50.

German foreign minister Sigmar Gabriel said the process would not be easy for either side and lingering "bad feelings" would be understandable. 

But he said this could not be allowed to affect the talks. 

And Austria's chancellor Christian Kern says clarifying the status of EU citizens living in Britain was a priority for his government.  

Laura Kuenssberg
BBC political editor

It's arrived. The letter has been handed over, so the process of our leaving the European Union is underway.

The prime minister described it as a "moment of celebration for some, disappointment for others", acknowledging for one of the first times the 48% who wanted to stay in the EU.

But rather than a breezy goodbye, the tone of Mrs May's letter was instead a respectful farewell.

It was a marked, and probably inevitable, change from her conference speech at the Tory get-together in October, and a sentiment more conciliatory even than her Lancaster House speech in January that set out her main priorities, and admitted, once and for all, that the UK would not stay in the single market. 

Read Laura's full blog

The Irish government has said it is unlikely that an agreement on the EU's future relationship with the UK will be reached for a "considerable time".

In a statement, it said the negotiations would be "very challenging" and Brexit would have "significant economic, political and social implications" for Ireland.

Emphasising the contingency planning that had been undertaken, it said its priorities were to protect the peace process in Northern Ireland, support trade, and maintain the Common Travel Area with the UK.