A foster father found guilty of sexually abusing young girls in his care already had a conviction for lewd and libidinous behaviour when he was approved to look after children.

Aberdeen City Council confirmed yesterday it was aware of the conviction when he and his wife were approved as foster parents in 1993 but did not consider he posed a risk.

However, he was deregistered in 2001 after a complaint by a youngster which was investigated but did not result in a prosecution.

William Alexander, 57, was found guilty yesterday of molesting three of his foster children in the 1990s. In June 1979 the van driver was found guilty of lewd and libidinous behaviour and practices with a youngster and was fined £80.

During his trial this week Aberdeen Sheriff Court heard Alexander abused two 14-year-old girls and one seven-year-old girl between 1995 and 1997. One of the teenagers was lying in her bed when Alexander sneaked into her room and sexually assaulted her. He similarly abused a second teenager and a seven-year-old girl on separate occasions at his home in Cairnfield Road in Aberdeen.

Fiscal depute Duncan MacKenzie told the jury: "You are considering whether the accused sexually abused these three complainers.

"The very nature of these charges are that they usually occur in circumstances where it's the complainers' word against the accused about what happened. This has been a difficult case and you have heard harrowing stories."

During three days of evidence one of his victims had told the court that she had been assaulted after having been taken home drunk. Mr MacKenzie said: "She told you in graphic details what was done to her in the bedroom by the accused and how she felt when that was happening.

"Drunk or not, she was fully aware of what was being done to her and by whom. Three girls, unknown to each other, were spoken to independently by police. Each complainer had their own reasons for not disclosing at that time or later."

The jury took 40 minutes to find Alexander guilty. Sheriff Colin Harris deferred sentence until April 5 for the preparation of reports and placed Alexander on the sex offenders' register.

Outside court neighbours shouted abuse at Alexander, who had a leather jacket draped over his head.