Denying another independence referendum if there is a majority of MSPs in favour of it would push more people to support separation, voters have been told.
Greens co-leader Patrick Harvie and SNP leader Nicola Sturgeon said some Scots would not welcome being told they could not have another vote by Westminster.
The two leaders made their comments during a heated Channel 4 News debate, also attended by Douglas Ross, Anas Sarwar and Willie Rennie, which regularly descended into a shouting match, with leaders repeatedly cutting across each other.
Mr Harvie said: “The UK simply saying ‘no you can’t, no you can’t, no you can’t’ will guarantee more and more people saying ‘yes we will’.”
The First Minister and SNP leader said she did not think the Prime Minister would block another referendum, adding: “You don’t have to listen too closely to the noises out of Westminster to know that the Tories have already moved on to how they block another referendum… to debates about how they rig the franchise and the timing of it.”
She added: “If Boris Johnson decides to stand in the way, Patrick is right, it will push more people towards independence, but he would have to ultimately take the Scottish Government to court to stop that, because we would seek to progress legislation for a legal referendum and if Boris Johnson accepted that, by definition that’s a legal referendum.”
Tory leader Mr Ross said the SNP was planning to hold an “illegal referendum” if the UK Government denied the powers required for the vote, citing SNP MP Richard Thomson, who said on BBC radio on Saturday “the Scottish Government has been quite clear that the other route to that is simply to go ahead with a referendum”, although he went on to say that any vote would be “consultative”.
When challenged on Mr Thomson’s comments by the Tory leader, the First Minister said she would “speak for myself”.
Mr Ross also said at the beginning of the debate he believed the Prime Minister when he denied he said he would “let the bodies pile high in their thousands” rather than impose another Covid-19 lockdown.
Scottish Lib Dem leader Mr Rennie was asked if the best way back into the EU was as part of an independent Scotland, but the North East Fife MSP said: “For goodness sake, no – we should learn the lessons of Brexit, not repeat them with independence.
“Breaking up long-term economic partnerships is chaotic, we’ve seen that.”
At one point, following an exchange on Scottish independence, Scottish Labour leader Mr Sarwar interjected: “We are still in a pandemic… can I just remind people?”
He added: “10,000 of our fellow citizens have lost their lives, and all you’ve had for 20 minutes so far, is politicians bickering in a studio, when people’s lives and livelihoods are still at risk.”
He added: “Would you not rather that Nicola Sturgeon and I were working together on ending child poverty rather than arguing about borders?”
The Labour leader went on to accuse the Scottish Tories of secretly wanting to keep the debate on Scotland’s constitution going, saying: “I know Tory MSPs who privately say they like the debate about the constitution, they like the debate about the referendum, because the only way they get Tory MSPs elected is to talk about division and sound like there’s a big division happening and they’ve got to stop a referendum.”
Mr Ross brushed off the comments, saying: “You should maybe listen to the Tories about getting elected because the Scottish Labour Party have lost votes and MSPs at every single election since devolution.”