Australia will be left with no submarines if it abandons the Aukus deal with the US and UK, a senior defence official has warned, declining to publicly countenance an alternative plan if Australia’s promised nuclear-powered fleet does not arrive under Australian command.

“Defence has been directed to pursue Aukus and we are pursuing Aukus and that’s our plan. I would not venture into the space about ‘Plan B’ or ‘Plan C’,” defence department deputy secretary, Hugh Jeffrey, told a Sovereignty and Security Forum in Canberra on Friday.

Australia’s decades-long $368bn agreement to acquire eight nuclear-powered submarines has attracted intense scrutiny over the opacity of the deal, and laggard rates of shipbuilding in both of the countries on which Australia is relying.

Australia has already given more than $2bn – of $9bn promised – to the US and UK to boost their industrial capacities to build more submarines, without any guarantees that submarines would be delivered.

At a Security and Sovereignty Forum at the National Press Club, event host Malcolm Turnbull asked Jeffrey what Australia would do if the promised US Virginia-class submarines, and the subsequent UK-designed Aukus submarines, did not arrive under Australian command.

“What is the government’s Plan B if we do not get any Virginia [class submarines] at all because the Americans are not producing enough for their own needs?” Turnbull asked. “What is the Plan B if we end up with no new subs and we’re left with the creaking hulls of the Collins?”

The Collins class is Australia’s current class of ageing conventionally powered submarines, whose working lives have already been extended far beyond forecasts.

Turnbull, the prime minister whose conventionally powered submarine deal with French giant Naval was torn up by the Morrison government in favour of Aukus, has been a consistent and trenchant critic of the Aukus agreement. He pointed to consistently slow shipbuilding rates in the US, and the restrictive legislation which would prevent America selling any submarines to Australia if it would “degrade the United States undersea capabilities”.

Retired rear admiral Peter Briggs told the forum the US and UK could not provide Australia with the promised submarines on time, and said Australia should abandon Aukus, and “turn back”.

Jeffrey, deputy secretary for strategy, policy and industry, countered that Australia had tried, and failed, previously, to obtain a fleet of submarines to replace the ageing Collins class.

“This effort under Aukus is the fourth, by my count, attempt to replace a submarine program that we began in the 1980s. Each effort, since then, to replace it has fallen afoul of domestic politics. Are we really thinking that this should be the fourth?

“If you really want to be in a position where we have no submarines then ‘turn back’. I do think, speaking as an apolitical public servant, we need to get out of this relentless politicisation of defence capabilities.”

“Forgive me if I’m cynical about these questions. I do think we need to get on with business.”

Jeffrey declined to comment on any potential alternatives to the Aukus plan.

“It’s not my job as a public servant to talk about ‘Plan Bs’, that’s the prerogative of government. Defence has been directed to pursue Aukus and we are pursuing Aukus and that’s our plan. I would not venture into the space about ‘Plan B’ or ‘Plan C’.”