Sir Keir Starmer has refused seven times to rule out doing a deal with the Liberal Democrats after the next election if Labour finds itself the biggest party at Westminster but short of an overall majority.
Speaking to Sky News political editor Beth Rigby, Sir Keir declined to say whether he would do a deal with Sir Ed Davey multiple times.
It comes after both parties won hundreds of seats in last week's local elections in England at the expense of the Conservatives.
Extrapolations from the council votes show that the swing, if replicated at the next general election, would not be great enough to get Sir Keir into Number 10 as the leader of a majority Labour government.
While he categorically ruled out joining up with the SNP to form a government, the leader of the opposition would not be as decisive about the Lib Dems as the question was "hypothetical".
Asked about the SNP, Sir Keir said: "Well, look, I'm going for an outright majority, and I'm often asked 'will you do a deal with the SNP?'
"And I've been absolutely clear, there are no terms on which we would do a deal with the SNP.
"I want to push on to a Labour majority."
Asked whether he would do a deal with the Lib Dems, Sir Keir said: "I'm not answering hypotheticals, but… we are aiming for a Labour majority.
"And that's what we're confident about, because you know, this set of local elections was a cry for change and Labour is the party that can deliver that change."
Beth asked multiple times if a deal with Sir Ed would be on the cards, with the Labour leader repeating that he wanted to "press on" or "kick on" - and that he wants a "Labour majority government" - but not saying no.
A Labour Party spokesperson later said that the party was not "contemplating a coalition because we're on course for a Labour majority government".
"We're not thinking about anything else because last week showed we don't need to," they added.
In total, Labour won 536 seats last week, the Liberal Democrats won 405 and the Greens won 241.
This came at the cost of 1,063 Conservative councillors, and 119 members of other parties and independents.
The way people voted was used to calculate an estimate of what could happen at the next general election - expected in 2024.
It showed that Labour could win 298 seats - its highest tally since 2005, but 28 short of a majority.
With the Lib Dems projected to get 39 seats, the two together would have enough for a majority in the House of Commons under these conditions.
The same projection showed the Tories would be down 127 seats at 238, with the SNP and other parties taking 75 seats.
Sir Keir's unclear answer leaves the door open to Labour working with England's third biggest party less than a decade after they were in government with the Conservatives.
On Sunday, the Lib Dem's deputy leader Daisy Cooper told Sophy Ridge on Sunday that her party's aim was to oust as many Conservatives as possible - but also did not rule out a pact with Labour.