Lopes said he was stepping down to prevent the Portuguese armed forces from being "weakened by political attacks," in his resignation letter cited by the Lusa news agency.

"I categorically deny having been informed of any cover-up operation to protect the perpetrators of the theft," he added.

Senior military and paramilitary police officials were arrested last month on suspicion of offences including arms trafficking, criminal association, abuse of power, and the falsification of documents, according to the prosecutor's office.

The arrests come more than a year after thieves broke into a military arms depot in Tancos, central Portugal, where security cameras had been out of order for two years.

Among those picked up, according to local media reports, was Luis Vieira, head of the military police.

They got away with 44 anti-tank rockets, 150 hand grenades, 18 tear gas grenades, and 1,450 nine-millimetre cartridges.

It was only an anonymous tip-off months after the July 2017 robbery that allowed police to recover most of the haul, which was hidden on waste ground a few dozen kilometres from Tancos.