Gary Voy denied committing the sickening offences when one of the girls was as young as three and the other six years old.

But after a six-day trial at Dumbarton Sheriff Court a jury of nine men and six women found him guilty by majority verdict.

The 42-year-old, a former pupil at Braidfield High School, remained calm throughout proceedings and when he gave evidence on Wednesday he flatly denied any suggestion that he had acted inappropriately towards them.

He admitted occasionally disciplining the youngsters when he was their babysitter but said it only amounted to a “slap on the thigh” and there was never any contact of a sexual nature.

However, the victims both described being subjected to a series of degrading acts and being warned by Voy that someone in their family would die if they told anyone about the abuse.

The women, who can’t be identified for legal reasons, only reported his crimes to police 20 years later after one of the sisters said she saw Voy transporting primary school children unaccompanied in his taxi.

Earlier in the trial the court heard from both victims who are now mothers, aged 36 and 32.

The older sister told the court she was visiting Gary Voy’s home with her mother when the abuse began in 1985, when she was six and Voy was 14-year-old. He initially tried to kiss her and that progressed to touching her private parts. This continued on a number of occasions over a two-year period at a house in Clydebank until it reached a point where the young girl would refuse to go to the address to avoid being in Voy’s company.

The jury heard that Voy then turned his attention to the younger sister who was subjected to his abuse from 1985 to 1992 when she was aged between three and 11-years-old.

The court was told that the abuse “destroyed” her and pushed her into a life of drink and drug abuse from the age of 11. She refused to go to school and was eventually taken into care before she tried to take her own life.

Earlier in the trial the mother of the victims told a court she was “horrified” when her daughter told her she had been abused by Gary Voy 10 years after the last offence.

The 59-year-old told the court her youngest daughter was a happy child until she reached the age of eleven when she “turned into a right little horror”.

She said her daughter sucked a dummy until she was 11 and that this had caused her to speak to the family GP about her concerns but she didn’t find out about the abuse until her daughter was 20.

Asked to describe how she felt when her daughter told her Gary Voy was to blame, the mother said: “I was horrified. I just could not believe it. I had trusted him to look after my baby. I was sick, physically sick.” Paul Mullen, defending, highlighted discrepancies between the oral evidence in court and the police statements they had given when they initially reported the allegations in 2012.

Mr Mullen questioned the sisters’ “credibility and reliability” and argued that both Gary Voy and his brother Mark Voy, who also gave evidence, were “speaking from the same hymn sheet” but said you could not say the same about the women.

He said: “If you look at the evidence and ask yourselves has the evidence been corroborated beyond all reasonable doubt then I would suggest it has not. If you come to that conclusion then you must acquit Gary Voy.” However, in summing up the Crown’s case, prosecutor Sarah Healing described Voy as a “manipulative, arrogant and over confident sexual predator” who “smirked his way through the evidence”.

Urging the jury to convict, she said: “There had been no fall-out with the family, there had been no fall out with Gary Voy and Gary Voy accepts that. Why would they put themselves through that for an allegation that was false?” After two hours’ deliberation the jury returned a guilty verdict on both charges.

Sheriff Thomas McCartney told Voy, who has no previous conviction, he had been found guilty of a serious crime and should be aware that the court would be considering a “substantial period of imprisonment”.

He deferred sentence until May 21 and called for a criminal justice social work enquiry report and a risk assessment. Voy was placed on the sex offenders register. His bail was continued.