A 13-year-old boy has been arrested on suspicion of sexually assaulting a four-year-old girl at a children's play centre.
The girl told her parents the assault happened while she was playing between 12.30pm and 1.30pm on Saturday.
The teenager, from Lancaster, London, was questioned by police and has been released on bail pending further enquiries.
Detectives are now appealing for any information about the alleged assault.
Detective Inspector Phil Jones, who is leading the investigation, said: 'This is a very sensitive investigation and we are seeking the help of the public.
'I would ask the parents or relatives of any children who were at The Zone on Saturday between 12.30pm and 2pm to contact the police.
'I would like to stress that at this stage, there is no suggestion that any other child has been a victim; we do believe this to have been an isolated incident.
'However, we are seeking any witnesses that may have seen something suspicious or out of the ordinary.
'The young girl and her parents are currently being supported by specially trained officers and The Zone is fully cooperating with our investigation.'