Nationalist MSP Alex Neil also suggested a campaign of civil disobedience was needed to force the UK Government into granting a Section 30 order, giving Holyrood the power to hold indyref2.
Boris Johnson last week confirmed he would not considering granting permission for a second referendum, and his Scottish Secretary Alister Jack has claimed even a pro-independence referendum at the 2021 Scottish Parliament election wouldn’t change the position.
Speaking on the BBC’s Politics Scotland programme, Mr Neil - a former Scottish Government cabinet minister - argued that the Scottish Parliament already had the power to hold a consultative referendum that would change the facts on the ground if it is won by the pro-independence side.
"The political reality is, if you run a legal, consultative referendum, and the result was in favour of independence, the game's a bogey for the Union, because the political reality is – that's it."
Asked what would happen if the pro-Union side boycotted a consultative referendum, Mr Neil said: "It's not a Catalonia situation where, according to the Unionists, the Spanish constitution made the Catalonian referendum unconstitutional and they boycotted it.
"There's a very big difference between boycotting an unconstitutional, illegal referendum and boycotting a legal referendum.
"So the point I'm making is, let's see if a consultative referendum, at the right time, could be legal."
He added: "Politically, I think it's a very important distinction, because if you decide to abstain in a legal referendum – as a third of the people do regularly in general elections, decide to abstain – it doesn't change the result.
"And the rest of the people who want to vote in a legal referendum, even those who do not want to boycott it but want to vote against independence – they can't be held to ransom because the Conservative Party and their cronies are going to abstain."
Asked what would happen if the UK Government never granted the powers for indyref2 to take place, Mr Neil compared the situation to independence movements in former Soviet republics and post-imperial India, saying: "They said that about the Baltic states in terms of the USSR.
"Of course we all know people pressure – remember the people holding hands between Estonia, Latvia and Lithuania?
"If it comes to that kind of demonstration by the Scottish people to force the issue, then that's what happens.
"If you look at India – India didn't have a referendum. It was things like the salt strike and things like that that forced the hand of the British Government.
"So we need to force their hand in other ways."