Theresa May has told the Cabinet that her letter to the European Union giving formal notification of Brexit will be "one of the most important documents in our country's recent history", Downing Street said.

The Prime Minister said it will "set the tone for our new relationship with Europe and the world", in a signal that it could be a substantial statement of the UK's intentions, rather than a simple message informing the EU that she is triggering Article 50 to begin Brexit.

Mrs May told ministers the invoking of Article 50 on March 29 would be a "historic event" which will see Britain begin a "bold new chapter as a prosperous, open and global nation", her official spokesman said.

It comes after European Council president Donald Tusk announced that EU leaders will meet on April 29 to collectively agree a Brexit negotiating position, pushing the start of substantial talks back to at least May.

Mrs May's spokesman said he would not go into the details of what will be in the letter until closer to the time, but added at a regular Westminster briefing: "The (PM's) words speak for themselves."

The PM also said the work of the Cabinet's EU exit and trade committee has "provided a strong platform from which we can now prepare to enter negotiations to secure a deal for the whole of the UK".

The spokesman said Mrs May expects the exit process to be "smooth and orderly", giving certainty to individuals and businesses and "allowing us to make preparations that will see an independent UK prospering outside of the European Union".

Tuesday's Cabinet was described as a "good and very positive discussion" with many contributions.

Next week's Cabinet meeting will be switched to Wednesday so ministers can discuss the letter on the day of notification, the spokesman added.

The spokesman did not say how the letter will be delivered to Mr Tusk, who has said he will respond within 48 hours.

EU leaders will then have around a month to consider the response before coming together at the extraordinary summit in April to agree their position and give European Commission chief negotiator Michel Barnier a mandate for exit negotiations.

The date of April 6 was initially pencilled in for a summit but it was pushed back after Mrs May delayed triggering Article 50 to the end of the month, despite receiving Parliament's approval to do so on March 13.

The timing of the summit means substantive negotiations will not begin until at least May, giving the PM just 17 months to get a deal, according to Mr Barnier's timetable.

The Frenchman has called for talks to be completed by October 2018 to give time for any agreement to be ratified before the UK leaves, expected on March 29 2019 under the two-year Article 50 process.

Mrs May's spokesman reiterated her intention to agree an exit deal within the timeframe.

"No change in our position; we intend and expect to achieve this within a two-year timetable," he said.

Mr Tusk said his priority was making sure the "divorce" was the "least painful" for the EU.

Giving a press statement in Brussels, he said: "In view of what was announced in London yesterday, I would like to inform you that I will call a European Council on Saturday April 29 to adopt the guidelines for the Brexit talks.

"As you all know, I personally wish the UK hadn't chosen to leave the EU, but the majority of British voters decided otherwise.

"Therefore, we must do everything we can to make the process of divorce the least painful for the EU.

"Our main priority for the negotiations must be to create as much certainty and clarity as possible for all citizens, companies and member states that will be negatively affected by Brexit, as well as our important partners and friends around the world."