Theresa May has now signed a letter to trigger Article 50, the start of the official process to leave the EU.
Tube Strikes Announced
Two 48-hour Tube strikes have been announced for February over plans to close all London Underground ticket offices.
Members of the Rail Maritime and Transport (RMT) union will walk out at noon on February 4 and again from the same time on February 11, threatening travel chaos for workers and visitors to the capital.
Station staff will stop work from noon on both days, while drivers will not book on for any shifts between the hours of the action.
The move followed a 77% vote in favour of strikes in protest at the closure of all 260 Tube ticket offices, with the loss of hundreds of jobs.
Members of the Transport Salaried Staffs Association (TSSA) will also vote in the coming weeks on whether to launch a campaign of action.
RMT general secretary Bob Crow said: "RMT members on London Underground have voted by a massive majority for both strike action and action short of a strike in a dispute which is wholly about cash-led cuts plans that would see the axing of nearly 1,000 safety-critical jobs and the closure of ticket offices at a time when the Tube network is under growing pressure from customer demand and needs more staff and not less to ensure safe and efficient operation.
"Not only are 1,000 posts on the line but staff remaining are going to be forced through the humiliating and degrading experience of re-applying for their own jobs - the same staff who have been hailed as heroes when the Tube has faced emergency situations.
"That is a kick in the teeth for the loyal and experienced Tube workforce who have kept services running safely and efficiently under constant pressure from weight of demand and a creaking and under-resourced infrastructure.
"These cuts would hit the vulnerable, the elderly, those with disabilities and women the hardest. De-staffing stations, with supervisors running operations three stops down the line on an iPad would turn the Tube system into a criminals' paradise where those with violence and robbery on their minds are given a clear run.
"RMT will work with our sister unions and passenger groups to ensure that Tube users understand just what's at stake as (London mayor) Boris Johnson turns his opportunist election pledges on their head.
"Before the Tories start shouting the odds, they should take note of the fact that the turn-out in this ballot was higher than the last mayoral and GLA elections and the vote in favour massively outstrips anything that those same politicians can even dream of in terms of a popular mandate."
TSSA general secretary Manuel Cortes blamed the ballot on the "reckless" behaviour of Mr Johnson, who he said was refusing to meet the unions over their genuine fears for safety and security.
He said: "It was the mayor who came into office in 2008 with a firm pledge to keep open every ticket office on the grounds of keeping passengers safe and secure at all times.
"Now he wants to scrap the lot, claiming there will be no problems because he will keep staff on station platforms - those that keep their jobs, that is."
Transport for London said sales at ticket offices accounted for only around 3% of all journeys, after announcing that staff would be switched to station concourses.
Phil Hufton, LU's chief operating officer, said: "All Tube stations will remain staffed at all times when services are operating, and we'll be introducing a 24-hour service at weekends during 2015. In future, there will be more staff in ticket halls and on platforms to help customers buy the right ticket and keep them safe and secure.
"We're committed to working with unions and staff to implement changes to station staffing without compulsory redundancies and we've been clear that there'll be a job for everyone at LU who wants to work for us and be flexible. I urge our trade union colleagues to work with us to shape the future of the Tube.
"I want this to be a real consultation and I'm determined to listen to all views expressed. However, threatening industrial action will be seen by hard-working Londoners and businesses as totally unnecessary, given all the commitments we have made."
LU said there had been a low turnout in the RMT ballot, which it maintained meant that nearly 70% of union members either did not want industrial action or refused to vote.
The company said every Tube station will be visibly staffed and controlled by LU staff during operating hours, there will be a job for everyone who wants to stay and is "ready to be flexible", and any operational changes will be done without compulsory redundancies.
A career in the media and entertainment industry starts here...
A man has been fined £1,800 for flying drones over Parliament, near Buckingham Palace and over football stadiums.
Government to investigate after some restaurants accused of withholding tips
Most Viewed Pictures On Heart
Recently Played Tracks
Now playing: Non-stop hit music
Deposit £10 to get a £40 Welcome Bonus - That's £50 to play bingo, slots and more!*
Over 50 tracks to make you feel-good. New album out now...
Find your local four day weather report here.
What's happening on the roads where you are?
Make Heart the soundtrack to your day and you could be a winner with great prizes up for grabs throughout the day.
Get some great ideas on how to advertise your business on radio, online and on mobile.