A woman was dragged to the ground and sexually assaulted on a footpath beside a busy main road in Hounslow.
The victim, who is in her 30s, was walking on the southern side of the A4 Great West Road, near the junction with Springwell Road, when the attack took place.
Her attacker is described as a white man, aged 35-40. He was said to be clean-shaven, had wavy black hair and was wearing a blue shirt, black trousers and a hip-length green jacket that evening.
The attack took place nearly a month ago, on Sunday (February 8) between 8.30pm and 8.40pm, but police are still hunting the culprit.
Officers believe at least two passing pedestrians witnessed the attack, and they have urged them to come forward. They are also keen to hear from any motorists heading towards Heathrow that night who may have seen something.
Anyone with information is asked to call police on 020 7161 9813 and to leave a contact number if the line goes to voicemail. Alternatively you can call Crimestoppers, free and anonymously, on 0800 555 111.