The realisation he had been engaged in a long-standing affair left her feeling unloved, unwanted and “raw inside”, she told Cambridge Crown Court.

She and her husband, John, 58, had not had sex for 15 years, she said, and he had told her he was impotent and that his condition could not be treated.

“I found it a hard thing to have a sexless marriage and I was so sorry for him,” she said.

But Mrs Foster’s sympathy for her husband turned to disbelief and anger when she discovered e-mails showing he had been having an affair with mature student Julie Simpson, 45 - for more than 15 years.

For Mrs Foster, a retired foot specialist from Bromley, south London, this was the ultimate betrayal.

John Foster met Julie Simpson in the 1980s while working at the BBC

The 61-year-old told Cambridge Crown Court she could not believe her husband would have sex with another woman after convincing her he was impotent.

Mrs Foster said she had long suspected the affair between her husband and Miss Simpson - a family friend - but that he had dismissed her fears, even giving the impotence drug he thought her unreasonable for voicing her concerns.

Mr Foster, a former BBC journalist for 35 years, and Miss Simpson had met in the 1980s when both had worked for the hypertension and impotence
in Westminster.

After finding e-mails confirming her worst fears, Mrs Foster arranged a meeting in the Cambridge college room of her husband’s mistress to confront her about the affair.

‘No warning’

And so it was that Mrs Foster, consumed by “misery and despair” at the breakdown of her marriage, arrived at Lucy Cavendish College on 3 October last year, with a kitchen knife hidden in her handbag.

I never meant to kill her. I never meant to hurt her. I meant to kill myself Alethea Foster

But her plan, she told Cambridge Crown Court, was to slash her own wrists in front of Miss Simpson if the meeting did not go well.

“I’ve been married to John for 35 years and I couldn’t imagine life without him. I never meant to kill her. I never meant to hurt her. I meant to kill myself,” she told detectives in interviews.

“I took a knife from my kitchen and if it was obvious after talking to her that nothing was going to go right I was going to kill myself.”

Miss Simpson, from Beckenham, south London, told the court that during their meeting “without warning” Mrs Foster climbed out of the chair and punched her in the stomach before attacking her with a knife.

She just looked very pleased with herself. Everything was gleeful really Julie Simpson

“She went down to the bag on the floor, then she was up out of the chair and she hit me. It felt like a punch.”

Miss Simpson told the court Mrs Foster was smiling.

“She just looked very pleased with herself.

“Everything was gleeful really.”

In all, Miss Simpson was stabbed 17 times and felt certain she was “probably going to die”.

Speaking after the verdict, Miss Simpson said her life had been saved by the “skill and expertise” of paramedics and the doctors at Cambridge’s Addenbrooke’s Hospital.

The undergraduate student lost the sight in one eye and suffered a punctured lung as a result of the attack.

Mrs Foster told police she could recall asking Miss Simpson questions but that the next thing she male impotence was being outside the room with a knife in her hand and Miss Simpson lying nearby.

It was on the basis that she had no memory of the attack, coupled with her claim she had taken the knife to kill herself, that Mrs Foster entered not guilty pleas to all the charges.

And on Friday, the jury at Cambridge Crown Court cleared Mrs Foster on charges of attempted murder and causing grievous bodily harm with intent.

They did, however, find her guilty of causing grievous bodily harm with no intent.