In a dramatic development at Antrim Crown Court, the convicted double murderer changed his plea on nine of 14 counts involving four women.
As the first charge was read, with a number of his victims sitting yards away in the public gallery, the former lay preacher spoke from the dock.
“Guilty, and I am sorry,” he said.
The 52-year-old, jailed for life last year after confessing to murdering his wife and his ex-lover’s husband, also admitted his guilt as charges relating to three more patients were read.
Some of his victims, who were in court to give evidence, wept and embraced relatives as Howell was led back to the cells at the end of the brief hearing.
Six women had claimed Howell attacked them in his Ballymoney practice over a 10-year period from March 1998 to December 2008.
Last year, he admitted assaulting three patients in his surgery.
Two of those three women alleged he attacked them more than once and those additional charges, along with claims filed by three other women — 14 counts in total — were due to be contested in the trial in Antrim.
Howell has now admitted to 12 of 17 charges involving five patients, but an alleged assault against a sixth woman, and four other charges related to additional allegations made by two of the five women, will remain on the books, said prosecution lawyer Neil Connor.
Last year, the dentist was sentenced to life in prison, with a 21-year minimum term after admitting he murdered his wife Lesley and Trevor Buchanan, the policeman husband of his then lover Hazel Stewart, in Coleraine in May 1991.
Stewart, aged 48, was also convicted of the double murder, which police had believed for 19 years was a double suicide, after standing trial earlier this year. She is serving a life sentence with a minimum of 18 years.
The bodies of Ms Howell and Mr Buchanan were found in a car in a fume-filled garage in the seaside town of Castlerock.
Howell had staged the scene after poisoning the pair with gas piped from his car exhaust as they slept in their own homes in nearby Coleraine.