Willie Rennie, the leader of the Scottish Liberal Democrats, has said there is a "distinct possibility" that Scotland could vote for independence unless the pro-UK campaign improves.

Speaking as party activists gathered in Aberdeen, Rennie said the anti-independence campaigners had to be far more positive about the UK's strengths and benefits by adopting a "sunshine strategy" in favour of the union.

"I think there's a possibility Scotland could be independent in September. I think there's a distinct possibility," Rennie told BBC Scotland. "That's why we need to focus really hard on what that means."

Rennie said the pro-UK campaign needed to switch emphasis from the negatives of independence to pursue a strategy that emphasises the positives of the UK. "We want to talk more about those things and less about the 'ah, buts'," he said.

"I'm hoping that Better Together [the pro-UK campaign] will embrace that sunshine strategy over the coming months. People need to know that there's something great about the United Kingdom. We need to remind them what that is."

Rennie added that attacks on the case for independence would continue: "It doesn't stop us asking difficult questions. We have a right to ask difficult questions without being accused of being negative, but nevertheless let's emphasis the great things about the UK too."

His warning came after a spate of opinion polls showed a narrowing in the gap between yes and no, and the pro-independence vote slowly gaining ground, with less than 200 days to go before the referendum on 18 September.

The Better Together campaign has been accused repeatedly by its critics and by some supporters, including Charles Kennedy, the former UK Lib Dem leader, of being too negative, with too many attacks over North Sea oil, the banks and currency.

Rennie's "sunshine strategy" is now a conference catchphrase, apparently to counter the repeated typecasting of the pro-UK campaign as "Project Fear" by independence campaigners after a leaked internal memo from Better Together used the phrase last year.

Evidence of a voter backlash was underlined in the latest poll by YouGov for the Times, which showed earlier this week that more voters believed that the Conservatives, Lib Dems and Labour were lying when they stated in February that a currency union would be vetoed by a future UK government.

The poll found that 45% of voters believed the veto was a campaigning ploy and that the UK government would agree a currency pact after a yes vote, and only 40% believed George Osborne, the chancellor, would stick to it.

The poll put support for independence two points up at 37% against 52% who would vote no.

It also found high levels of negativity on the issue of independence towards Alistair Darling, the Better Together chairman and former Labour chancellor, and David Cameron, the prime minister. Darling had a -24 (minus 24) trust rating and Cameron a -49 trust rating, against a -19 trust rating for Alex Salmond, the first minister.

The Daily Mail reported that Better Together had convened a crisis meeting by the organisation's board to discuss the slip in the polls on Friday, and debate a new, more positive strategy – a report denied by Better Together officials.

One source insisted the meeting was solely to rubber stamp a pro-UK, anti-independence advertising campaign to be launched by Better Together in April, with the campaign's spending boosted by new donations.

But the official admitted there had been internal debate about how heavily that campaign would stress the downsides of independence, with some strategists disputing the need to change tactics.

Nick Clegg, the UK Lib Dem leader, is due to firm up Rennie's tactic by urging the no campaign to make the case for remaining with the UK "just as thrilling as the drama of leaving it" in his conference speech on Friday afternoon.

Clegg will tell those behind the no campaign they have a duty to prove that Scotland will have new and exciting opportunities if they vote against independence.

"Forgot hearts and minds. This referendum is about capturing imaginations," he will say, according to advance extracts of his speech.

"Just as it is right that we must explain the risks and consequences of Scotland voting to leave the union, we must also set out the opportunities of voting to stay. The prospect of remaining in the UK must be just as thrilling as the drama of leaving it."