Gordon Brown and Alistair Darling have urged pro-union voters in key Scottish seats to back Labour at the general election to "stop the SNP".
The ex-prime minister and his former chancellor reunited to pen letters to almost 100,000 Scots in constituencies where Labour believes it can oust the SNP, calling on them to help prevent "another divisive referendum" on independence.
Nicola Sturgeon says that if the SNP win a majority of Scottish seats in theJune 8 vote, they will have earned a triple democratic lock after winning the Holyrood election last year and receiving the backing of MSPs for their referendum plans in March.
The First Minister has repeatedly insisted that Scotland "must have a choice" about whether to become independent in the wake of Brexit.
But in his letter, Brown called on Sturgeon to "focus on the day job of running our schools and hospitals, and forget about her obsession with another divisive independence referendum".
Former Better Together campaign chief Alistair Darling warned voters that a "vote for the Conservatives, or any other party, won't stop another divisive referendum, it will simply help the SNP win here and across Scotland".
The SNP described Brown and Darling's message as "a sad, dreary intervention from two of yesterday's men" and said Labour were privately "cheering on the Tories" in certain seats.
The remarks come after an STV News report revealed that unionist parties in marginal constituencies are working together to defeat the SNP by tactically limiting campaigning and fielding "paper candidates" to help the best-placed pro-union candidate.
Brown and Darling's letters are being delivered to voters in Edinburgh South, East Lothian, Lanark and Hamilton East, Kirkcaldy and Cowdenbeath, Coatbridge, Chryston and Bellshill, and Midlothian.
Darling has already penned a letter to voters in East Renfrewshire with a similar message.
In the recent local elections in May, Labour were the largest party in East Lothian and Midlothian, and believe they are better placed than any other pro-union party to defeat the SNP in the other seats.
Brown wrote: "The SNP has been in government for more than ten years but is so obsessed with independence that our cherished public services are deteriorating.
"I was proud to play my part in the campaign to remain in the UK.
"It is time for the SNP to use the powers of the Scottish Parliament to improve standards in our schools and hospitals, rather than taking us back to the arguments of the past."
The former PM added: "No matter which party you normally vote for, it's only a vote for your Scottish Labour candidate that can stop the SNP here."
Darling told voters: "A vote for the Conservatives, or any other party, won't stop another divisive referendum, it will simply help the SNP win here and across Scotland.
"Don't wake up on the morning after election day to the SNP celebrating a general election victory they will use to continue their march towards independence."
The SNP said a vote for Labour in Scotland risks letting Tory MPs in "by the back door".
A party spokeswoman said: "This is a sad, dreary intervention from two of yesterday's men - Scotland has moved on, but neither Gordon Brown or Alistair Darling seem to realise it.
"People in Scotland remember them as the cheerleaders for the Tory-led Better Together campaign.
"Indeed, Labour are working hand-in-glove with the Tories in councils across Scotland - and they have privately admitted that they are cheering on the Tories in seats across Scotland.
"The truth is that Labour would be willing to see a Tory government with an increased majority just to spite the SNP.
"Labour cannot win this election in Scotland and any votes for them risk letting Tory MPs in by the back door - the fact that Gordon Brown and Alistair Darling cannot even mention Jeremy Corbyn in their desperate letters shows they have no faith in his abilities."
The Scottish Conservatives and Scottish Liberal Democrats have been approached for comment.
You can find a full list of candidates standing in Scotland for the general election here: