The Inside Out programme, broadcast on BBC East and BBC North on Monday night, also reports that thousands of pounds are wagered on these races.

The BBC team filmed groups taking part in a race on main roads in Hertfordshire, County Durham and Cumbria and talked to participants in the trotting races.

It also revealed that top horses can fetch as much as £20,000 at markets such as the Watton Horse Fair in Norfolk.

Illegal racers hold up traffic near Durham

Mark Clarke, a road racer, told the BBC that trotting races, with harnessed carriages, are a regular sight on roads in England and large amounts of money are wagered.

"Races are happening every week. The police can't stop you," he said.

"Some people race for thousands, some people race for £9,000 or £10,000."

Race organiser Joe Crombie said bets of up to £30,000 are made during races in County Durham and in Penrith, Cumbria.

Tyrone Roberts, an auctioneer with the Watton Horse Fair, said finding horses at auction for races was part of the travelling community's traditions.

Horse auctioneer Tyrone Roberts at Watton

"It's part of there way of life. They deal in a particular type of horse, mainly the coloured variety," he said.

"They race these carts and horses up and down the roads and it can be quite dangerous."

The programme filmed races on a dual carriageway in Hertfordshire and a single carriageway near Durham.

They involved horse-drawn carts, with race followers driving next to, in front of and behind the racers.

After the race finished the drivers scattered after hearing a police siren, although Hertfordshire Police did not arrest anyone.

The race was run on a dual carriageway

A spokesman for Hertfordshire Police said: "It's not an issue for us here in Hertfordshire.

"They have a right to use the road as much as anyone else.

"At the end of the day it's their own animals they are putting at risk."

Mr Crombie said: "It's all done by phone and it's all about who has the best horse.

"We always have at least two cars - one and the back and one at the front - to make sure the horses can get up some speed."

Harry Harbour, who trains horses for legal harness racing, defended the tradition of road-racing.

"It's not cruel at all. It's in the horse to do it," he said.

But the RSPCA said police should stop the races.

"The activity by its very nature will compromise the welfare of the horses," said a spokesman.

"We are concerned not only that animals are endangered but also about the danger to the public.

"If people want to race horses they should do it on legal race tracks with vets present."