The former head of the Royal Navy, Ben Key, has been fired and stripped of his commission after it was discovered that the married admiral had been having an affair with a female subordinate.

The Ministry of Defence said Key’s behaviour had been found to have fallen far short of the standards expected, after an investigation triggered when the woman’s husband made a complaint.

The head of the armed forces and Key’s immediate boss, Adm Tony Radakin, said: “We expect the highest standards of behaviour from our service personnel and our civil servants.

“We investigate all allegations of inappropriate behaviour and will take robust action against anyone found to have fallen short of our standards, regardless of their seniority.”

Key acknowledged in a statement that his behaviour had fallen short. “I deeply regret my conduct in the spring of last year, which fell well below the standard I set for myself and that which I set for the Royal Navy,” he said.

The former admiral said he fully accepted the decision, made by the defence council, to terminate his service and his commission. “I am very sorry to those I have hurt personally, and I apologise to everyone who serves with the Royal Navy,” he said.

Key, 59, had been forced to step back from all duties in May while the complaint was investigated.

The case had caused considerable unease in the Royal Navy, not least because Key said recently he was determined to root out sexual misconduct in a service that has been hit by a string of scandals.

“We are absolutely determined to create a Royal Navy in which people are judged for their professional conduct,” he told the defence select committee in March. “Behaviours that run counter to that will not be accepted, and particularly those around unwelcome sexual behaviours.”

Key had also offered an apology for the “intolerable” misogyny suffered by women in the submarine service, after a series of investigations had exposed sexual harassment, bullying and assault.

He said last October that he was “truly sorry” to women who had experienced “misogyny, bullying and other unacceptable behaviours” while serving their country. “We must be better than this and do better than we have,” he said.

Sophie Brook, one of the first women to be allowed to serve in the submarine service, spoke out in October 2022 about sustained and aggressive sexual harassment during her time in the navy. It had resulted in her self-harming to the extent that on one occasion she required stitches, she said.

Women remain a significant minority in the navy and the Royal Marines, accounting for 11.3% of those serving, according to diversity statistics published in April. That is higher than the army and slightly lower than the proportion of women who serve across the military, which is 11.9%.

A married father of three, Key joined the navy in 1984 and was appointed first sea lord in November 2021. Last month, Gen Sir Gwyn Jenkins, a former marine and chief of UK special forces, was appointed to replace Key.