A garage worker who killed three of his partners over four decades must serve a longer minimum jail term after appeal judges ruled his original sentence was unduly lenient.

Theodore Johnson, 64, was sentenced in January to life in prison with a minimum of 26 years for the “brutal and merciless” murder of his most recent victim, Angela Best.

Johnson bludgeoned Best with a claw hammer and strangled her with a dressing gown cord in north London a couple of months after the end of their 20-year relationship.

He had two previous convictions for manslaughter for killing two other former partners, in 1981 and 1993. After the attack in 1993, he was detained in a secure hospital but was conditionally discharged in 1997.

At the Royal Courts of Justice on Thursday, three court of appeal judges increased his minimum term to 30 years after the solicitor general, Robert Buckland QC, argued his original sentence was unduly lenient. Johnson will be 94 before he is eligible for parole.

Outside court Buckland said the judges’ decision was a “proper reflection of what was a most appalling set of circumstances that led to the murder of the defendant’s third partner”, adding: “I’m very glad the court agreed with my submission that the minimum term should be increased to reflect the extreme gravity of the case.”

Buckland said protecting women was “paramount” to the current government’s considerations.

Johnson was born in Jamaica where he grew up as one of 11 children working on a banana plantation. He moved to the UK in 1980 with his then wife, Yvonne Johnson, who he killed in 1981 after an argument. He hit her with a vase before pushing her off the balcony of their ninth-floor flat in Wolverhampton. He was convicted at Stafford crown court in November that year.

He went on to kill Yvonne Bennett 11 years later, strangling her with a belt at their home in Finsbury Park, north London.

In March 1993, at the Old Bailey he was convicted of killing Bennett by diminished responsibility. He was suffering from a depressive illness and personality disorder and received a secure hospital order.

In 1994, a period of both unescorted and escorted release commenced, the court heard. In 1996, his application to a mental health tribunal for a conditional discharge was refused. He applied again in 1997 and his application was approved, although he was ordered to disclose all future relationships as a condition of his release.

Increasing Johnson’s sentence, with Best’s family looking on in the public gallery, Lord Justice Holroyde said: “The obvious purpose of that condition was to safeguard women. We found nothing in the evidence about Mr Johnson’s condition to suggest he was not aware of the importance of the condition.

“The awful reality is Ms Best might be alive today if Mr Johnson had complied with that requirement.”

Johnson denied being in a relationship to his medical and social supervisors on numerous occasions: in 2006, 2011, 2015 and the same month he murdered Best.

Best ultimately discovered by chance details of Johnson’s past after finding letters that detailed one of the killings, prompting him to reveal both of his previous convictions.

Johnson attacked her when she visited his flat in December 2016. The couple had recently separated and Best had started a relationship with another man.

Hours after killing her, Johnson threw himself in front of a train at Cheshunt railway station in Hertfordshire, losing his right arm and left hand.

He appeared in court via video link, slumped in a wheelchair in grey prison-issue clothing, and showed no emotion as his sentence was increased.

“The learned judge failed fully to reflect the true seriousness of his crime,” Lord Justice Holroyde said of the initial sentencing.