Former prime minister Gordon Brown has issued a late plea for Unionist voters to back Labour, insisting it is the only way to prevent "another divisive referendum" on independence.

The ex Labour leader has reunited with chancellor Alistair Darling, with both men penning letters to almost 100,000 Scots in seats where the party believes it has a chance of ousting the SNP on June 8.

Mr Brown told people that "no matter which party you normally vote for, it's only a vote for your Scottish Labour candidate that can stop the SNP here".

Voting for either the Tories or the Liberal Democrats "won't stop another divisive referendum", Mr Brown claimed.

"It will only help the SNP win here and allow Nicola Sturgeon to use the result to continue her campaign for independence."

The former prime minister is credited with playing a key role in the successful campaign to keep Scotland in the Union in 2014 as the author of "the vow" - a commitment that Scotland would get more powers if it rejected independence that was signed by the leaders of the three main unionist parties.

Less than three years on from the 2014 referendum, Mr Brown said it was important to send First Minister Nicola Sturgeon "a message that she should focus on the day job of running our schools and hospitals, and forget about her obsession with another divisive independence referendum".

The former prime minister said: "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."

Lord Darling also had a high-profile role in the fight to keep the UK together as chairman of the cross-party Better Together campaign.

He told voters: "I retired from frontline politics after the independence referendum but I'm sending you this letter because we have a real opportunity at the ballot box on 8 June to end talk of another referendum.

"People voted for Scotland to remain in the UK and for a stronger Scottish Parliament.

"This has been delivered and the Scottish Government now need to get on with the day job and use the powers of the Scottish Parliament to build a country fit for the future instead of taking us back to the arguments of the past."

Lord Darling continued: "It is becoming evident that the SNP's obsession with independence is having a harmful effect on the Scottish economy.

"A combination of shrinking growth and lack of inward investment means the Scottish economy is lagging well behind the rest of the UK.

"Regardless of who normally gets your vote, it's only a vote for your Scottish Labour candidate that can stop the SNP here.

"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."

He told people: "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.

"Your cross in the box of the local Scottish Labour candidate at this election will go a long way to getting the message across that we don't want another referendum. Make your voice heard."

A spokesman for the SNP 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 Alasdair Darling cannot even mention Jeremy Corbyn in their desperate letters shows they have no faith in his abilities.

"Now more than ever, it is vital to elect strong SNP voices standing up for Scotland at Westminster."