Scotland’s deputy first minister has accused George Osborne of making “the wrong moral choice” on nuclear weapons after the chancellor announced £500m of extra spending at the Trident submarine base in Faslane.

John Swinney accused Osborne of jumping the gun by making the announcement before the Commons could have a full debate on Trident’s renewal and the future shape of the UK’s defences.

Osborne, who was visiting the base on the shores of Gare Loch in Argyll and Bute on Monday, announced a further £500m to upgrade and expand its infrastructure, including jetties and seawalls. All the UK’s nuclear and conventional submarines will be based there from 2020.

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Osborne said the investment was a dividend for Scotland remaining part of the UK. He also claimed that a Labour party led by Jeremy Corbyn, allied with the anti-nuclear Scottish National party, would pose a threat to national security by undermining the future of Trident.

Speaking on BBC Radio Scotland, Swinney said the SNP believed “there should be a full, open and democratic debate” about the purpose of nuclear weapons.

This debate would “determine defence priorities for a very long time indeed [but] many people in this country have a fundamentally different moral and strategic argument about whether to renew Trident or not.”

Swinney said the case against Trident was clear, particularly when welfare spending was being cut so heavily.

Facebook Twitter Pinterest George Osborne (rear left) arrives by boat at the jetty at the Royal Navy’s submarine base at Faslane. Photograph: Russell Cheyne/PA

“When we step back from all of those questions about defence, of course within the budget priorities of the UK choices have been made and the chancellor is making a choice to essentially prioritise investment in nuclear weapons over the protection of the most vulnerable citizens of our country,” Swinney said.

“We believe that is the wrong moral choice to be making given the hardship individuals are facing in our country today.”

A final Commons vote on renewing Trident is due in 2016 and the debate is expected to be one of the most divisive yet on British nuclear weapons with 56 SNP members having been elected on a strong anti-nuclear platform, and a resurgence of anti-nuclear sentiment in the Labour party with Corbyn’s leadership campaign.

It could also increase political divisions between Scotland and the rest of the UK – a fear likely to have strongly influenced Osborne’s decision to press the case for new jobs and investment at Faslane on an official visit before the Commons resumes.

As it stands, only two of Scotland’s 59 MPs, the Tory Scottish secretary, David Mundell, and the former Scottish secretary Alistair Carmichael, support nuclear weapons: Ian Murray, Labour’s shadow Scottish secretary and its sole MP in Scotland, has said publicly he will vote against renewing Trident. Swinney said a number of Tory MPs also preferred higher spending on conventional forces to new nuclear weapons.

In an article for the Sun to coincide with his announcement, Osborne said a Labour party aligned with the SNP on Trident would threaten the country’s security. He said that Corbyn’s candidacy should not be treated as a joke.

“On the contrary, I think we should take it deadly seriously,” he wrote. “For the new unilateralists of British politics are a threat to our future national security and to our economic security. We’re going to take on their dangerous arguments and defeat them.”

Facebook Twitter Pinterest HMS Victorious, a Trident submarine, on patrol off the west coast of Scotland in 2013 before a visit by David Cameron. Photograph: AFP/Getty Images

The only breakdown in agreement over the need for a nuclear deterrent was during the 1980s when Labour was dominated by the left, he said.

“Now that consensus, which is so important for our security and reliability as an ally, risks being shattered again by an unholy alliance of Labour’s leftwing insurgents and the Scottish nationalists.”

Osborne claimed the new investment would secure 6,700 jobs at Faslane allowing it to prepare for the arrival of all the Astute-class conventional submarines and the second-generation Trident fleet. By 2022, there would be 8,200 civilian and military jobs there.

Speaking on BBC Good Morning Scotland, Osborne said the base was “a massive source of jobs and investment” and insisted the government had a mandate to prepare for Trident’s replacement.

“First of all, a majority of MPs in the House of Commons voted in the past to renew Trident and the vast majority of MPs now, a vast majority, have been elected on manifestos to renew Britain’s nuclear deterrent to keep our country safe, to provide that ultimate insurance policy, to make sure we remain a free democracy that can defend itself at the last instance,” he said.

“And I think we should be completely unapologetic about making that argument and taking on those who would leave our country defenceless.”

He said Nicola Sturgeon, Scotland’s first minister and SNP leader, and Alex Salmond, the party’s foreign affairs spokesman at Westminster, should be challenged on whether they could guarantee the UK would be safe from attack in 35 years’ time, when the next Trident fleet would still be operational.

“I don’t think that they can because we do live in a dangerous and uncertain world, and I want to make sure that all people in this country have that ultimate security and guarantee,” Osborne said.

Swinney appeared to suggest the money saved from scrapping Trident’s renewal could be invested in better conventional forces, contradicting the SNP line on using the money for non-military spending. He said the UK should spend 1.7% of its GDP on defence, less than the 2% Nato target for its member states.

The Scottish Tories sought to further divide Labour by accusing the Scottish Labour party of a “deafening silence” about Osborne’s announcement on Monday, and of reversing its previously resolute support for Trident under previous Scottish party leader Jim Murphy.

His successor Kezia Dugdale, who personally supports retaining Trident, has opened the way for the party’s left to reverse Scottish Labour policy on the deterrent at its next conference in an effort to acknowledge the strongly anti-nuclear view of MSPs, shadow Scottish secretary Ian Murray and party activists after Corbyn’s surge in the leadership polls.

She has cleared the way for a full conference debate on disarmament, in a move also designed to dilute the SNP’s attacks on Labour policy.

Jackson Carlaw, the Scottish Tory deputy leader, said: “Only this time last year Labour politicians such as Jackie Baillie rightly warned about the damage a Yes vote [in the referendum] would do to Faslane’s future and to the economy of the west of Scotland. Yet today, instead of welcoming this fantastic investment in Faslane’s future, so far we hear deafening silence from Labour.”