Andy Burnham is facing a Labour backlash as he hints at going 'all the way' and pledging to rejoin the EU.
The PM used his big conference speech yesterday to blame Brexit for many of the country's problems, saying he will spell out options for the future at a summit with Brussels later this year.
But the move has sparked concerns among his own troops, with Scottish Labour leader Michael Marra warning returning to the bloc is 'unrealistic'.
The stance also represents a dramatic U-turn from May, when Mr Burnham told a conference in Leeds: 'I am not proposing that the UK considers rejoining the EU.
'I respect the decision that was made at the referendum and it is going to undermine everything I have said about strengthening democracy if we don't respect that vote.'
In interviews after his speech yesterday, Mr Burnham told the BBC he did not want to leave the issue 'hanging' and he wanted to be clear on 'where we want Britain to go in the next decade'.
Asked whether he wanted the UK to rejoin the bloc, he told Radio 4's Today programme he wanted to 'look at the options'.
Andy Burnham is facing a Labour backlash as he hints at going 'all the way' and pledging to rejoin the EU
'We could stay as we are. That's definitely an option, if people think this is the right place to stay,' he told the programme.
'We could look at what (former Conservative chancellor) George Osborne has said about a customs union, we could look at what the Liberal Democrats said at their conference about the single market or we could go all the way.'
Labour's election manifesto in 2024 ruled out joining a customs union, going back into the single market or allowing the return of freedom of movement with the EU.
But Mr Burnham has indicated he is prepared to recommend going beyond those red lines in future.
'There are clearly options that need to be considered, and we have to look at what is doable and what would be the pros and cons of each,' he told the BBC.
He said the need for a debate about the UK's future relationship with Europe was part of his plan to shift the country in a new direction.
'We've been in a rut. The country hasn't been working for people as it should,' he told BBC Breakfast.
Tory Deputy Leader Alex Burghart said: 'You cannot trust a word Andy Burnham says.
'Just a few weeks ago he said he wanted to move on from the Brexit arguments and now he is dragging Britain back into them by floating the prospect of rejoining the EU.
'Labour cannot blame a democratic vote from over a decade ago for the consequences of their own economic decisions. While they look backwards, the Conservatives will look forwards.'