Former Scottish first minister Alex Salmond said a second referendum on Scottish independence could be held in two years time.
In a column in the National newspaper, Salmond said he was in little doubt the next test was coming and that he anticipated a second referendum for the autumn of 2018.
Mr Salmond resigned as SNP leader following defeat in 2014's independence referendum.
However, opposition parties have been quick to speak out against the plans, reminding the former first minister that the SNP had said the last independence referendum was "once in a generation".
Salmond wrote: "If I have to guess a date then I'd say that it is likely to be the autumn of 2018. And even on that two-year timescale there is much to be done.
"In the first vote, what began as a Scottish government initiative transformed into a popular campaign. We should never forget that inspiration.
"However, indyref2 can't just be the same. As Leicester seem to be finding out in the Premier League, much to the regret of this romantic football fan.
"You can't roll out the same tactics and trust them to be as effective as before."
Nicola Sturgeon, who succeeded Alex Salmond to become First Minister in 2014, shares his views on a second referendum calling it "highly likely" following the UK's vote in favour of leaving the EU. Brexit was widely opposed in Scotland, while it was supported in England and Wales.
However, Sturgeon is also said to be exploring "options" to maintain Scotland's links with the EU, as part of the UK's Brexit negotiations.
Writing in The Times, Scottish Labour leader Kezia Dugdale said: "There should be a renewed focus on the day job from the SNP.
"Two years on, the divisions caused by that historic contest have not healed. On social media and in the real world, the scars that are Alex Salmond's political legacy remain painfully obvious.
"Salmond and Nicola Sturgeon promised voters that the result would stand for a 'generation'.
"I believe that when politicians promise something, the voters have a right to expect them to honour that. Yet here we are again, two years later, and Nicola Sturgeon is agitating for another referendum."
The Scottish Conservatives have also been vocal in opposing a second Scottish independence referendum.
Leader Ruth Davidson said: "Scottish Conservatives are out in towns and cities across Scotland today to stand with all those who don't want our country dragged back to a second referendum.
"Two years ago, we settled this issue – we decided to remain within the United Kingdom.
"This anniversary should be the moment when we move on, and we will stand with all those people across the country who want to do just that.
"We call on the SNP today to respect the decision we made. There is no justification to drag us back to an argument that belongs in the past.
"But the truth is that, far from listening to Scotland, it is the Scottish Nationalist Party that is proving itself to be deaf to Scotland's interests."