Mountain bikers are being warned to be on their guard after Americans who came to the Forest for the Wild Boar Chase had cycles worth about £20,000 stolen.

One of the bikes was an irreplaceable prototype which is one of just two in the world.

Two members of staff from American company Transition Bikes were among 400 riders taking part in the Wild Boar Chase.

Before the 42-mile race the Americans and a UK distributor called into Monmouth to look around the town and the offices of Dirt magazine.

While there, four bikes were stolen from a rack on the back of their van which was in a car park in the centre of town.

Police believe gangs are deliberately circling the Forest and the Wye Valley because the area is becoming known internationally as a mountain biking hotspot.

They are convinced thieves are watching popular biking spots looking for top of the range cycles worth thousands of pounds.

Some are opportunist thieves who snatch bikes from racks on cars or cycles left unattended.

But police are also convinced that thieves are following the owners of high value machines home so they can return at a later date to steal them from garages or sheds.

Supt Phil Haynes fears the problem could get worse when the warm weather arrives.

He said: "This is a nationwide problem because criminals have cottoned on to the fact that this is a very popular sport and the value of the bikes is very high.

"It's important that police and bike owners work together to prevent these crimes taking place.

"The criminals know exactly where mountain biking is taking place in areas like the Forest so people need to take sensible precautions.

"They should try to avoid leaving their bikes on the back of vans or cars and be vigilant about who is following them home."

Mike Rose, Dirt Mountain Bike magazine editor, said it's not just about money and there is a huge emotional attachment between a rider and their bike.

"There seems to have been a spate of bike thefts recently, and many of these bikes are worth thousands of pounds. "Thieves these days know the value of these bikes and will seemingly do almost anything to get their hands on them, and they know all the tricks.

"I really do think that it is time for the bicycle industry to seriously look into the use of GPS tracking equipment to find these bikes once they have been taken".