The world's oldest roadworthy car is scheduled to chug through Sussex tomorrow - but it could be touch and go.

The Grenville Steam Carriage is one of 400 vehicles taking part in the annual London to Brighton car run.

Its wheels were made at the Great Western Railway workshops in Swindon in 1875 and it looks more like a Victorian railway engine than a car.

If the steam carriage makes the 60 mile journey from Westminster to Madeira Drive via Croydon, Crawley, Cuckfield, Burgess Hill, and Preston Park, it will be the longest journey the contraption has ever made.

The Grenville weighs more than two tons and requires a crew of three people to drive and keep the steam pressure going. It has a top speed of 20 mph and cruises at between 10mph and 12 mph.

Colin Wilson, spokesman for the RAC Motor Sports Association, which organises the run, said: "It has not gone 60 miles before and it will do well to reach Crawley, but you never know. "

More than a million spectators are expected to line the route, including thousands who will watch the cars as they reach Brighton seafront.

Personalities taking part include Nick Mason, drummer with Pink Floyd, Lord Montague of Beaulieu, and Prince Michael of Kent.

All the vehicles taking part were built before January 1905 and the second oldest vehicle taking part is an 1892 Peugeout Vis a Vis.

The run celebrates the coming into law of the Locomotives on the Highways Act on Saturday November 14, 1896, which raised the national speed limit from four miles per hour to 14 mph. The Act also abolished the requirement for all road vehicles to be preceded by a man in front with a red flag.

The cars set off from Westminster at 7.35am and the frontrunners are due to reach Crawley by 9.10am. The first of those that manage to complete the trip are expected to arrive in Brighton at 10.40am.