UP TO six fire engines were rushed to Ascot Racecourse last night after a fire broke out in a service tunnel.

Dense black smoke filled the tunnel and crews were worried the blaze may spread up into the Grandstand above less than three weeks ahead of the venue's stellar event.

Firefighters from Bracknell and Ascot were amongst those called to the iconic racecourse at 11pm.

Crew manager Steve Borham from Bracknell fire station said a faulty thermostat sparked the fire.

"There was a lot of heavy smoke and all sorts of plastics," he said.

"When the smoke cleared out, and that took quite a while, the thermal imaging camera showed quite a lot of heat and there were concerns about it spreading up into the Grandstand.

"Luckily it didn't spread anywhere else. It was a good job to stop that because you don't want it getting up to the grandstand a couple of weeks before Royal Ascot.

"The fire had a bit of an impact on the racecourse's water supply but they are working on that today (Sunday) and tomorrow.

"It is thought a thermostat for quite a big water tank caused the fire."

Thousands of punters along with The Queen, other royals and a host of celebrities will descend on Ascot for the week-long racing spectacle next month.

Top jockeys will compete in one of the highlights of the racing calendar. It is not believed the fire will have any impact on the racing.