Labour leader Jeremy Corbyn has urged MPs from all parties to install him as prime minister, in order to stop a possible no-deal Brexit.
So, where does the party stand on Brexit?
A temporary government
Mr Corbyn’s 14 August letter restates the party’s opposition to a no-deal Brexit.
He says he will call a vote of no confidence in the government. And if it succeeds, he hopes to head a temporary government that would try to extend Article 50 – delaying the Brexit date – to allow a general election to take place.
In that election, Labour would argue for a “public vote” – a referendum – on the terms of leaving the European Union, in which voting to remain in the EU would be an option.
Brexit or no Brexit?
In Mr Corbyn’s letter, he stops short of saying the party would campaign for Remain if it was in power and there was a referendum.
But the party has previously indicated it would campaign for Remain in any new referendum held while the Conservatives were in power.
It is not yet clear whether any Labour government would try to reopen negotiations with the EU over a deal to leave, or proceed straight to a referendum.
The party told BBC News it would decide this when it produced its manifesto, which it will need to publish if a general election is called. A manifesto sets out what a party would do if it got into power.
Until July, Labour had resisted outright support for another referendum. Its deputy leader, Tom Watson, said “ambiguity” over the party’s Brexit policy had cost it votes at the European elections in May.
But delivering a referendum before 31 October, the date of the UK’s scheduled departure, will be all but impossible. An extension would be necessary to have a general election and then a referendum afterwards.
Labour’s previous position
Mr Corbyn has said he will call the vote of no confidence at the “earliest opportunity when we can be confident of success”.
If a no-confidence vote passed, by getting a majority of MPs to support it, Labour and other opposition parties would have 14 days to form an alternative government. If successful, Boris Johnson would be expected to resign as prime minister.
Labour’s previous Brexit plan was to leave the EU but maintain a very close trading relationship, by staying in a customs union and keeping close alignment to the single market.
That would mean the UK would be able to continue trading with EU without tariffs (taxes on imports) being applied.
It would also reduce the need for checks, making it less likely an Irish backstop would be needed. The Irish border problem has been a major sticking point for Brexit.
However, being in a customs union would prevent the UK from striking its own trade deals with other countries on goods – such as the US.
Coping with divisions
Just like the Conservative Party, Labour has had to deal with internal divisions over its Brexit policy.
Many Labour MPs who represent parts of the country where most people voted Leave are unhappy with the party’s shift on supporting a referendum.
More than 25 Labour MPs wrote to Mr Corbyn in June, saying another public vote would be “toxic to our bedrock Labour voters”. They urged the party leadership to back a Brexit deal before 31 October.
On the other hand, senior figures, such as Tom Watson and shadow foreign secretary Emily Thornberry, have been vocal advocates of another referendum for some time.
Mr Watson has also called on his party to work with the Lib Dems in a joint attempt to block a no-deal Brexit. This is despite the fact the Lib Dem leader, Jo Swinson, has ruled out an alliance with Jeremy Corbyn.
So, despite Labour now having a clearer position on a new Brexit referendum, questions remain.