A Labour government would not back a second Scottish independence vote during its “first term” in power, leader Jeremy Corbyn has said.
Scottish First Minister Nicola Sturgeon wants to hold a new referendum in 2020.
She has said her SNP MPs would only consider putting Mr Corbyn in Downing Street in a hung parliament if he accepts the “principle” of such a vote.
But Mr Corbyn said this would not take place in his first five-year term, when his focus would be on “investment”.
Prime Minister Boris Johnson has warned that a Labour-SNP alliance would “ruin 2020” with two referendums.
The Labour leader was speaking in Glasgow at the beginning of a two-day campaign visit to Scotland which will conclude with a rally in Edinburgh on Thursday.
Ms Sturgeon has said the SNP could back a Labour minority government in the event of a hung parliament on an “issue by issue basis”.
However, she has insisted that Mr Corbyn should not “pick up the phone” to ask for SNP votes unless he is ready to accept the “principle” of a second independence referendum, as well as ending austerity and offering further powers to Holyrood.
Labour has previously indicated that it would not actively block an independence vote, but would not make it an immediate priority when in power.
Mr Corbyn told journalists in Glasgow that there would be “no referendum in the first term of a Labour government, because I think we need to concentrate completely on investment in Scotland”.
It has previously been suggested that Labour would not oppose a referendum if the SNP won a majority in the Holyrood elections in 2021.
A party source said they would absolutely rule out a poll in 2020, and repeated Mr Corbyn’s claim that Labour would win the 2021 Scottish election.
The Labour leader said his party would “provide the massive investment Scotland deserves”, saying the general election was a “very simple” choice between the Conservatives or Labour being in charge.
He said: “I believe the people of this country deserve better than Boris Johnson and his government, and the country would be better off with a Labour government.”
Ms Sturgeon later told the BBC that Mr Corbyn was “weak” on many issues, and said Labour were not competitive in many Scottish constituencies.
She said: “The key thing in this election is the only party that can beat the Tories in Scotland is the SNP.
“We are the main challenger in every Tory-held seat. A vote for Labour risks letting in the Tories.”
The SNP leader said her MPs would stand up to the Conservatives as well as keeping a potential Labour UK government “on the right track”.
More clarifications to come?
Analysis by BBC political correspondent Nick Eardley
Labour has sometimes struggled to keep its message on Scottish independence clear and consistent.
The party in Scotland had said it believed Labour would have a mandate to reject calls for another referendum if it won power in Westminster.
But earlier this year John McDonnell made clear the party in London would not block a referendum if that’s what the Scottish Parliament wanted (and Holyrood does have a pro-independence majority).
Labour then said it wouldn’t allow a referendum in its formative years – assumed to mean before the next Scottish Parliament election in 2021.
Now, however, Mr Corbyn has muddied the waters suggesting he might not allow one at all in the first terms of a Labour government – in theory five years.
That calls into question the relationship between Labour and the SNP – who have said an independence referendum should happen next year. Nicola Sturgeon has said she won’t even speak to Mr Corbyn unless he allows the Scottish parliament to make the decision.
It’s possible there will be more clarifications to come from Mr Corbyn.
Prior to the campaign event, Mr Corbyn was heckled by a Church of Scotland minister who branded him a “terrorist sympathiser”.
As Mr Corbyn was telling reporters about a scarf given to him by a charity group, Richard Cameron, the minister at the local Scotstoun Parish Church, shouted that he thought the Labour leader would be wearing an “Islamic jihad scarf”.
He added: “Who’s going to be the first terrorist invited to the House of Commons when you’re prime minister?”
A spokeswoman for the Church of Scotland rebuked Mr Cameron, saying: “Whilst we may occasionally robustly challenge policy issues with which we disagree, we always intend to do that in a way that is polite and measured and allows for reasoned debate.”
The Conservatives and the Lib Dems have both positioned themselves in opposition to a second independence referendum.
Speaking at an electric vehicle manufacturer in the West Midlands later, Boris Johnson will say everyone in the UK faces “a historic choice” on 12 December.
The PM will say: “At this election, the country can either move forwards with policies that will deliver years of growth and prosperity, or it can disappear into an intellectual cul-de-sac of far-left Corbynism.
“We can honour the wishes of the people, or else we can waste more time, at the cost of a billion pounds per month, and have two more referendums, one on Scotland and one on the EU – an expense of spirit and a waste of shame, more political self-obsession and onanism.”
Scottish Lib Dem leader Willie Rennie, meanwhile, said his party “will never support another independence referendum”.
Speaking at a nursery in Dunfermline while promoting his party’s childcare policy, he said: “It’s really important we move on, we learn the lessons from Brexit rather than trying to repeat them with independence.
“Let’s try to tackle things like childcare expansion, let’s tackle the climate emergency, deal with mental health service problems that we’ve got in this country. We’ve had enough of the division and damage over the constitution.”