John Healey has criticised Labour figures jockeying to become prime minister in a politicised speech in which he said the party’s “very credibility“ in government was at stake if the infighting deepened.

The defence secretary, a Keir Starmer loyalist, said the party had turned in on itself since the May elections in what appeared to be direct criticism of Andy Burnham, Wes Streeting and even the junior defence minister Al Carns.

Arguing that “these are serious times”, Healey – who was also a minister under Gordon Brown – said Labour had to demonstrate how it would meet the moment while Russia continued to attack Ukraine and “crisis after crisis” hit British families’ incomes.

“We must not throw away so lightly the power we were given,” he said, contrasting his personal style with those hoping to oust the beleaguered Starmer as prime minister.

“Politics to me is not about the individual. I don’t much care about photo opportunities or PR firms. People will not forgive us if they think we’re more concerned about ourselves than we are about them,” he said at a speech to the Good Growth Foundation.

“Right now, the very credibility of Labour in government is at stake,” he said. “We must get serious. It’s not about us, not about the insiders of politics, it’s about the interests of the country.”

He did not mention others by name, but it was clear whom he was referring to. Allies said his speech was intended to be “a hard message” to the party, which should be seeking to get on with government.

Burnham, the mayor of Greater Manchester, who is standing for the newly vacant seat of Makerfield, has been pictured jogging at least twice since he indicated he wanted to return to Westminster and run to be prime minister.

Streeting resigned as health secretary last week in the hope of sparking a leadership contest, complaining that “where we need vision, we have a vacuum”. His initial bid stalled as it became clear he had little prospect of winning a contest.

Carns, a former special forces soldier, has claimed thousands of pounds of parliamentary expenses to produce promotional videos, including one showing him doing pull-ups at a fire station in competition with a firefighter.

Healey said defence spending had grown under Labour and would rise to 3.5% of GDP by 2035. The Ministry of Defence “estimates this means over half a million Brits will work for a defence firm in a decade’s time”, he said.

Future military spending would need to work “once for national security and once for British industry,” he said, setting a new guideline and arguing that defence jobs were skilled, unionised and offered an average salary of £57,000.

Despite the pledge, however, no date was announced for the publication of the long delayed defence investment plan, which has been the subject of a financial battle with the Treasury. Covering all critical military programmes, it remains up to £18bn overspent.

Healey promised the plan was coming soon, but said it had to be properly budgeted for. “Countries that cannot pay their way, cannot defend themselves,” he said, justifying the need to boost economic growth to pay for new arms.

He also sought to distinguish Labour’s approach from its nearest political rivals. He accused Reform of being supported by “pro-Russian cronies and crypto billionaires”, the latter a reference to Christopher Harborne, who donated £5m to Nigel Farage before he stood at the general election.

At the same time, he accused the Greens, who made sweeping gains from Labour in inner London in the local elections of wanting “to shut down the defence industry”.