A police sergeant put a female officer over his knee, spanked her and told her she was a "naughty girl" on a night out, a court heard yesterday.

Mark Downing, 39, also slapped her on the bottom at a barbecue and joked: "If it's there - it's only fair that I hit it," it was claimed.

Downing, who works for Thames Valley Police, denies three counts of sexual assault on two women officers.

Christopher Smyth, prosecuting, said colleagues had stopped inviting him to social events because of his inappropriate sexual behaviour.

The first two incidents happened during a drunken barbecue at one of the women's houses in Slough on July 6 last year, Swindon Crown Court heard.

Downing is said to have stood behind the woman - whose bottom he had allegedly spanked on another night - and commented it was "still perky".

Mr Smyth said: "She was shocked and he said that "If it's there - it's only fair that I hit it"."

The woman - who cannot be identified for legal reasons - said: "We had a good relationship at work but he changed when he got drunk and his behaviour was inappropriate."

Describing the earlier "spanking" incident, for which Downing is not facing charges, she added: "Mark was messing about and said I was being naughty and he put my legs over his and smacked my bum."

Later in the evening - after a vodka drinking game - Downing is said to have grabbed a second woman by the waist as she cleared up.

He grunted at her and told her "you and me outside" before being pushed away, the court heard.

This woman also complained that Downing later put his hand up her top during another night out - the third alleged offence.

The court heard that the women had a good working relationship with Downing, who is based at Slough police station.

Downing told the court that he could be "an idiot" when drunk and was "mortified" to have caused offence.

"When I do drink I become tactile but not when I'm sober," he said.

"I would have been mortified if I was told I had been acting inappropriately. It was a "hello" pat on the bum.

"There was nothing sexually derived from it at all."

He said he regretted doing it and denied grabbing the other woman around her waist on the same night or slipping his hand under her top on another occasion.

Downing, of Tilehurst, Berkshire, denies three counts of sexual assault.

The trial continues.