A dog became unwell after picking up a tennis ball thrown by a stranger, police have said.

Hertfordshire Constabulary says it is investigating the ‘suspicious incident’ which occurred in Cassiobury Park, Watford, on Wednesday, September 16.

The force says that a woman had been walking her dog in the park at 7pm when she was approached by a man who tried to talk to her about her dog.

In a post on Facebook, which has got more than 1,000 shares, a woman alleges that the man said "how beautiful and sexy" the dog was and that "he was asking some very odd questions".

Police said the man threw a tennis ball, which the dog retrieved before returning to its owner, police said.

Later that evening the dog became unwell at home – and the following day its owner contacted police.

The Facebook post said the dog had been up three times overnight with diarrhoea following the incident and had been sick.

The local Safer Neighbourhood Team has been informed and will be conducting enquiries.

Mayor of Watford Peter Taylor said: ""We take safety in our parks extremely seriously and will be working closely with the police regarding this serious allegation.

"Council staff and Park Rangers will be on high alert in the coming weeks to ensure everyone can continue to enjoy Cassiobury Park and feel safe."

Police are encouraging anyone who feels they have experienced a suspicious incident to report it.

Anyone can call the non-emergency number 101 or report information online at herts.police.uk/report.