Brexit: How could another referendum on leaving the EU work?
The former prime minister Tony Blair has renewed his call for another referendum - if there is no parliamentary majority for any one Brexit outcome.
"Parliament's gridlocked, Parliament can't decide and therefore we've got to go back to the British people to decide", the former Labour leader told Radio 4's Today programme.
Labour's official position is to try to force a general election if Theresa May's deal is rejected. If that attempt fails, the party says that supporting another referendum could be an option.
However, the prime minister has ruled out the prospect of another public vote. Mrs May has repeatedly told MPs that the 2016 referendum result "should be respected".
But what if MPs did decide - as Mr Blair is calling for - to have another referendum to break the deadlock over Brexit?
The Electoral Commission has told BBC News it has "contingency plans in place" and is ready "to respond quickly to any unscheduled poll".
The clock is ticking...
The UK is due to leave the EU on 29 March 2019. With 15 weeks to go, time is the most pressing issue.
If Parliament decided it wanted to hold another referendum, legislation setting out the rules of the poll and the regulation of the campaign would also need to be approved by MPs and peers.
It took seven months before Parliament signed off the previous legislation which led to the 2016 referendum.
But could it be done more quickly this time?
One possible option is to use the 2015 Act as a template and, in effect, copy over large chunks to speed the process up.
"This could be done very quickly in theory," says Alan Renwick, deputy director of the Constitution Unit at University College London.
If it were shortened in this way, Mr Renwick estimates it would still take about 11 weeks to get through Parliament.
Based on that timetable, it would take until late February for the bill to be passed - and that's if the process started now.
And it would be made much more time-consuming if MPs wanted multiple options on the ballot paper, as opposed to the "Leave" or "Remain" choice in 2016, adds Mr Renwick.
In the end it would be up to Parliament to ultimately decide what the question might be from a range of possible options.
The People's Vote campaign, which wants a referendum on the final Brexit deal, says a choice between Theresa May's deal and remaining in the EU is their preferred choice but it doesn't rule out the possibility of voters having three options to choose from.
Some argue that if there was to be another vote, 'Remain' should not appear on the ballot paper and it should be a straight choice between Theresa May's Brexit deal and leaving the EU with no deal.
There are other options as well. Justine Greening, a former Conservative cabinet minister who supports another referendum, has previously called for three choices when people come to vote:
- accept a negotiated Brexit deal
- stay in the EU
- leave with no deal
If there were several options, MPs would also need to decide what voting system to use - ie would voters make one choice or vote in order of preference?
The Electoral Commission would also need to test the proposed question and ensure it was presented "clearly, simply and neutrally".
Official campaign groups would also have to be selected.
The Commission then would need to provide information to voters about how to take part in the referendum and ensure there were enough counting officers in place around the country.
Once that's out of the way, there's the campaign - which usually lasts four weeks - and the vote itself.
The Electoral Commission told BBC News all of this would take a minimum of 10 weeks - from passing the legislation to holding the vote, as set out in the Political Parties, Elections and Referendums Act 2000.
That suggests it is highly unlikely that both the legislation and campaign process could be completed by 29 March 2019 - the UK's scheduled leaving date.
The 10-day referendum
But it's not unheard of for countries to hold referendums on much tighter timescales.
Three years ago, Greece organised a referendum in just over a week, in which voters rejected the terms of an international bailout following the country's debt crisis.
However, Alan Renwick says that if referendums are organised too hastily, it can give the impression that "normal procedure is not being followed" and that voters may see the eventual result as illegitimate.
A referendum similar to Greece's timescale, for example, would not allow enough time to organise postal votes and assess the question on the ballot paper.
Article 50 extension
The UK could ask the EU for an extension of the Article 50 period - which started the clock ticking on the UK's two-year departure from the EU - to allow more time to organise a new referendum.
But any attempt to extend it beyond 29 March 2019 would need to be unanimously agreed by all other 27 EU member states - according to Catherine Barnard, professor of European law at Cambridge University.
The EU has signalled it may be willing to allow the UK to delay its departure, but only if political circumstances changed (like allowing for a general election or a new referendum) - not just to give more time to renegotiate the existing deal.
The UK Parliament would need to give its consent too.
But that would mean cancelling the entire Brexit process - not just delaying it.
So in the event the UK wanted to hold another referendum, it could seek an extension to Article 50 first - with the agreement of the other EU member states
Then, depending on the result of the vote, the UK could then decide whether to revoke Article 50 unilaterally.
The alternative would be to hold the referendum after the UK is scheduled to leave but that could cause considerable practical difficulty - especially if the UK voted to be an EU member, after it had already left.