David Cameron's interference on the issue of Scottish independence has backfired by boosting support for a break from the Union, Alex Salmond has claimed.

A Sunday Express poll suggested for the first time since the SNP came to power that a majority of people (51%) back independence, and the Scottish first minister said Cameron's "dictatorial" intervention had won people over to his cause.

"I think that some of that increase in support for independence is a reaction against the sort of dictatorial line we've been getting from some of the pronouncements from Downing Street," said Salmond.

"I think the prime minister would do well to perhaps listen to the voice of the people and try to conduct this debate with a bit more positivity."

Despite his criticism of Westminster, Salmond raised the prospect of Scotland remaining part of the UK, even if people vote for independence, emphasising that the vote was about political autonomy.

"The Queen will still be our head of state … I don't think it would be a good idea to talk about United Kingdoms when what we're actually talking about is political independence for Scotland," he said.

Salmond brushed off a threat by the Scottish entrepreneur Michelle Mone to move to London with her business if there is a yes vote in the referendum. Mone, the co-owner of MJM International and creator of the lingerie brand Ultimo, told the Sunday Times (paywall): "I don't think we can survive on our own and I think it would be really bad for business."

The first minister said Mone had made the same threat in 2007 before the SNP took power and had not acted on it. He said her fear that business taxes would go up was misplaced. "It's very important … we have an extremely competitive rate of corporation tax, that's our policy," he said.

Salmond defended the wording of the referendum question – "do you agree that Scotland should be an independent country?" – as "fair and decisive", and backed the possible inclusion of an additional question on so-called devo-max, which involves more economic rather than political independence.

"If the demand is there, I think it would only be inclusive and democratic to allow that voice to be heard," he said.

Cameron, supported by the Liberal Democrats and Ed Miliband, has said there should be no option other than full independence on the ballot paper in autumn 2014. The prime minister has warned Scotland that a substantial increase in financial powers would be inconsistent with staying in the UK, Scotland on Sunday reported.