A British couple who abused a child and streamed the assaults live are beginning lengthy jail terms after a fellow paedophile who watched online from the US gave evidence against them.

Sarah Gotham, 34, was jailed for nine years on Thursday after a jury at Plymouth crown court convicted her of six counts of sexual assault between December 2012 and March 2013. Her co-accused, Craig Forbes, 36, who had earlier admitted three counts of sexual assault of a child, was jailed for eight years.

Gotham had denied the charges against her but a key prosecution witness, Kori Ellis, 42, a convicted sex offender, told the court she had watched Gotham abuse a girl alone as well as with Forbes.

The pair drugged and sexually abused the girl, sending images to Ellis in California via Skype. Ellis was arrested in San Francisco in April 2015 as part of an FBI investigation. She confessed to being a paedophile and to contacting others online. She agreed to cooperate with officers in return for a reduction in her sentence.

“After I got arrested I felt bad about what I did and I feel I should make sure that other kids don’t get hurt,” she said.

Ellis contacted the defendants through a Chatroulette-style site, Omegle, and they swapped Skype details. She said Gotham and Forbes, who were sexual partners, drugged the girl to abuse her, and that she watched.

After Ellis told the FBI about her contact with Gotham and Forbes, its officers alerted Devon and Cornwall police, who launched an investigation. In what is thought to be a legal first, four FBI officers travelled to the UK to give evidence in person to a British court.

Gotham, a retail sales assistant from Plymouth, hid her face behind a scarf as she was jailed for nine years. The judge Ian Lawrie QC told her: “On many levels it is painful to have to sentence you to a period of custody because it is clear that until your late twenties you led a hard-working life.

“You cannot sidestep the fact you played an equal part in the pursuit of Mr Forbes’s interests. You fell under the corrupting influence of Mr Forbes. But that does not excuse your responsibility for the offences. I am sure the consequences of the sentence you receive today is clearly a very painful lesson.”

Forbes, from Torquay, was handed an extended sentence, which means he will serve at least two-thirds of his eight-year term before he can be released and will spend another eight years on licence.

The judge described him as an arch-manipulator of both men and women and said he held a “malign and corrupting influence” over Gotham.

“It is clear there is little that can stop you when you wish to commit sex offences,” said Lawrie. “I do take the view that you are a dangerous man and you present a constant and persistent danger to women and children.”

Both were placed on the sex offender register for life and made the subject of sexual harm prevention orders until further notice. Three other counts of sexual assault of a child and one count of rape will lie on file against Forbes.

DS Brian Slade, of Devon and Cornwall police, said: “Gotham and Forbes carried out horrendous acts on a vulnerable child. While Forbes admitted what he had done, Gotham has acted completely without remorse and has always claimed that she had nothing to do with it.

“Forbes is clearly a very dangerous sex offender. However hard we find it as a society to believe that females can enter into such actions for their own enjoyment, the fact of the matter is that there is no excusing that both Gotham and Ellis are paedophiles who acted purely for their own pleasure.”