The Scottish Conservatives have urged discontented Labour voters who oppose Scottish independence to back the Tories, as they accused Jeremy Corbyn of planning a post-election coalition with Nicola Sturgeon.

Jackson Carlaw, the Scottish Tory interim leader, urged unionist Labour voters who had not backed the Tories before or were thinking of sitting this election out to vote tactically against Corbyn.

Speaking at the launch of his party’s Scottish manifesto, Carlaw said: “I say to all pro-union voters who are thinking hard what to do in this election, voters who might be thinking of giving it a miss, or voters who haven’t voted for us before: ‘Forget Scottish Labour, they have abandoned the union, and Jeremy Corbyn has abandoned you.’

“Instead, lend us your vote, join with us, because this is important. Together let’s make Nicola listen for once and together we can move on as a country.”

That theme was echoed in a rambling speech at the manifesto launch by Boris Johnson, who repeatedly referred to a “Corbyn-Sturgeon coalition” – an arrangement the Labour leader has repeatedly rejected.

Johnson said the two leaders were plotting to revoke his deal with the EU, hold two referendums in 2020 on staying in the EU and then on Scottish independence, and planned to weaken the UK’s armed forces.

“It is absolutely farcical that the nation’s time, the nation’s economy, should be put at risk by these proceedings next year,” he told supporters at a hotel in North Queensferry, about a mile from the home of the former Labour prime minister Gordon Brown.

“It would be farcical if the consequences weren’t so disastrous for the UK; the jobs not created; the investment in this country forestalled; the opportunities lost as a result of Corbyn’s dither and delay.”

Labour has explicitly denied that it would hold two referendums in one year, ruling out another Scottish independence vote before 2022.

Boris Johnson at the launch of the Scottish Conservatives manifesto in Inverkeithing. Photograph: Murdo MacLeod/The Guardian

Johnson made several unforced errors in his speech, first by wrongly claiming that a proposed spaceport in Sutherland, in the far north of Scotland, was under construction. It has not yet been approved by the government or received planning permission.

He then said Sturgeon had admitted in a BBC interview with Andrew Neil, broadcast on Monday evening, that she wanted an independent Scotland to join the euro. The SNP said that was false.

And in what appeared to be a confused reference to Corbyn’s personal support for unilateral nuclear disarmament and his refusal to confirm he would use nuclear weapons, Johnson claimed: “Corbyn has said he wants to scrap our armed services, and can’t think of circumstances in which he would use them.”

Corbyn has endorsed Labour’s policy of supporting renewal of the Trident nuclear weapons system, while including the UK’s arsenal in multilateral disarmament talks. He has also accepted that a Labour government could in future grant Holyrood the power to hold a fresh independence vote; Sturgeon believes he may be forced to do so next year, in exchange for SNP support for a minority Labour government.

The Scottish Tories are expected to hold most of the 13 seats they won in the 2017 election but are braced for some losses, including in Stirling, which they expect to be won back by the SNP, and Aberdeen South, where the former Tory MP and candidate stood down after being accused by a Labour MP of sexual harassment in the Commons.

The Tories are pessimistic about their chances of winning Perth and North Perthshire from the SNP. It is Scotland’s second-most marginal seat, with an SNP majority of 21 votes, but the Brexit party is still standing a candidate there, splitting the pro-Brexit vote.

As in previous elections, the Tories believe they can attract tens of thousands of unionist Labour voters, often Orange Order members or those with family members in the armed forces, who are angry at Corbyn’s Irish republican sympathies and his ambivalent stance on Scottish independence.

Carlaw told reporters the Tories would not agree to any fresh independence vote for a generation, a time period he said could stretch to 40 years. He said the SNP’s prospectus for independence, Scotland’s Future, had promised the 2014 referendum was a once-in-a-generation event. That document also warned that if the Tories won in the 2015 general election, they could stage a referendum on leaving the EU.

“I don’t believe in a culture of disposable democracy, which the SNP now subscribe to, where you can say that you’re going to have a once-in-a-generation referendum and then within a matter of a few years, say that you need to have that whole process again,” Carlaw said.