THE son and heir of the Earl of Caithness was placed on the sex offenders' register yesterday after being convicted of having underage sex with his girlfriend.

Alexander James Berriedale, 23, whose father is a former government minister and close friend of the Prince of Wales, was seen having sex last year with the 15-year-old girl at a beauty spot near Wick.

Wick Sheriff Court was told yesterday that he was still in a relationship with the girl, who is now 16.

She attended court in support of her boyfriend.

Berriedale - whose mother, the Countess of Caithness, committed suicide 10 years ago - was placed on the sex offenders' register for one year.

Despite the consensual nature of the earl's relationship, his name will appear alongside the likes of Ian Huntley, the Soham killer, and Roy Whiting, who previously had been convicted for the kidnapping and assault of a child, and who was on the register when he murdered Sarah Payne.

Sheriff David Sutherland ordered two special conditions. The young aristocrat - who holds the title Lord Berriedale - was told he must undertake a joint sex offenders' awareness programme and also make a donation of (pounds) 500 to the Crisis Pregnancy Centre, in Inverness.

Gary Aitken, procurator-fiscal, said Berriedale committed the sex offence at the access road to the outdoor swimming pool at the Trinkie, on the outskirts of Wick, last October 12.

He added: ''This was seen by another person. Police were contacted and they carried out an investigation.

''He was interviewed under caution and was entirely straightforward and co-operative. He admitted what had happened and that he knew the girl's age.''

Berriedale, of Hollandmake, Barrock, near Wick, originally had denied the charge in court. However, he changed his plea to guilty last month.

Sentence had been deferred until yesterday for the preparation of social inquiry and community service reports.

Neil Wilson, defending, said a social worker gave his client a positive report which ''adequately sets out his background and the difficulties he has''.

He added: ''The important thing to stress is he maintains a relationship with the girl, who is now over 16.''

Sheriff Sutherland told Berriedale that he would be placed on the sex offenders' register for one year.

Berriedale is the son of Malcolm Sinclair, the 20th Earl of Caithness, a former government minister and a close friend of the Prince of Wales and Camilla Parker Bowles. The Marlborough-educated earl spent much of his childhood at Balmoral, and frequently played with Prince Charles and Princess Anne. He was the princess's escort and dancing partner at her first dances and society balls when she was 17.

He once was paymaster general and was created a privy councillor in 1990. He was also a regular visitor to the Queen Mother's home at the Castle of Mey in Caithness.

He has been in the House of Lords since 1984 and has been a spokesman on health, social security, Scotland, transport, the Home Office and the environment, as well as serving as a whip.

The earl resigned from the post of transport minister in the Lords in 1994 after Diana, his wife and Berriedale's mother, killed herself at the family's home in Oxfordshire.

She was reported to have shot herself just before their nineteenth wedding anniversary.

The Caithness line goes back to 1455.

The son is the heir to the Sinclair clan's ancestral home at Girnigoe Castle in Caithness, which the family is hoping to preserve for (pounds) 1m.

He attended the (pounds) 2500-a-term Maidwell Hall School in Northampton, and has a sister, Lady Iona Alexandra, who was born in 1975.