The red Transit van was stolen from Grendon Lakes, near Wellingborough, at about 2pm on Thursday while the owner was fishing.

The van contained building materials, fishing equipment and a wallet.

Lakes site supervisor Paul Green said: "It was a crime of opportunity because the van was unlocked. We had spotted those responsible at the site but they weren't acting suspiciously enough for us to approach them before it happened."

He warned people using the lakes to be aware they are an open site, accessible to anyone. But Mr Green also said there were CCTV cameras and staff monitoring the car park 24 hours a day, and this was the first vehicle theft at the lakes in three years.

A silver Volkswagen Golf was spotted driving off at speed at the same time as the Transit van left the lakes.

A police spokesman said: "Theft from cars at country parks and other beauty spots traditionally increases at this time of the year. We are keen to warn people that if they don't want crime to spoil their fun, they should leave nothing in their car when they park it."

Staff at other parks are also aware of the problems.

Jacquie Grocott, a ranger at Stanwick Lakes, said: "We are trying to encourage people not to leave valuables on show. The police have been here drawing people's attention to it recently.

"They have walked round seeing people leaving their belongings on show and they are trying to stop it happening."

A police spokesman said: "We would also encourage people to park in the designated car parks, not on the road where cars have also been targeted in the past."

Last year a police survey of three town centre car parks in Wellingborough found 137 vehicles with valuable items on public display.

Witnesses to the van theft should contact police on 08453 700700.