Douglas Ross, the Scottish Tory leader, has said his party would boycott a Scottish independence referendum set up without the approval of Westminster. Ross said Scottish National party proposals for Holyrood to empower itself to stage a fresh referendum if Nicola Sturgeon’s party wins a majority in May’s election should not be seen as credible or legitimate. He urged Labour and the Liberal Democrats to reject any attempt by the SNP to sidestep the current legal arrangements. Under the Scotland Act 1998, Holyrood can only hold an official referendum after being granted a section 30 order by Westminster. Ross said on Monday that Sturgeon had until now promised she would only support a fresh vote on that basis; it was the process in the 2014 referendum, and one she had described as the “gold standard”. Describing any other route as a “wildcat referendum”, Ross said: “Anything that constitutes [an] unofficial referendum should be boycotted. It shouldn’t be given any credibility. It is again a divisive tactic by the SNP to split our country apart.” He added: “I would take no part in that. And I would hope anyone – not just unionist supporters – but people who support democracy, should not take part in these wildcat, unofficial referendums. “So, yes, I would make that plea to Scottish Labour, the Scottish Liberal Democrats and anyone who believes in democracy in Scotland.” Sturgeon told BBC1’s Andrew Marr Show on Sunday that if the SNP won a majority in May’s elections, based on a manifesto commitment to hold a referendum, her party would have a clear democratic mandate. “The polls now show that a majority of people in Scotland want independence,” she said. “If the SNP win the Scottish election in a few months’ time on proposition of giving the people that choice, then what democrat could rightly stand in the way of that?” The SNP published a pre-election blueprint at the weekend proposing that if Westminster ignored the election result and continued to refuse a section 30 order, Holyrood would legislate to hold an indicative referendum itself. This policy, which is likely to trigger legal challenges that would end up in the UK supreme court, is expected to be central to the SNP’s May election manifesto and adds to pressure on the Tories and Labour to offer an alternative. Many Labour MSPs believe the party should support a referendum if the SNP wins a majority – a position rejected by Keir Starmer and the Scottish Labour party’s leadership. Both parties are edging towards proposing a UK-wide constitutional convention or commission but have so far failed to set out their proposals in detail. The former Labour prime minister Gordon Brown added to that pressure on Monday by urging Boris Johnson to set up a constitutional commission to investigate wholesale changes to the UK’s structures. Writing in the Telegraph, he said: “I believe the choice is now between a reformed state and a failed state. [It] is indeed Scotland where dissatisfaction is so deep that it threatens the end of the United Kingdom.” The dispute raises significant questions about the legality of holding a referendum without a section 30 order. Holyrood’s presiding officer would rule on whether the bill was competent; ministers could push it through without that endorsement but then would face a legal challenge in the supreme court. Using public funds to draft the legislation and stage one could also be beyond the Scottish government’s powers Rona Mackay, an SNP MSP, said Ross’s boycott calls were “Trumpian”. She said: “Opposition parties have a right to oppose independence – but they certainly do not have a right to deny the Scottish people the chance to determine their own future. “The Tories are utterly terrified of the verdict of the people, but this Trump-like attempt to deny the results of free and fair elections will not stand.”