Motorists are facing a year of misery as work gets under way on a £150?million improvement scheme on the M6 through Birmingham.

A 50mph speed limit, which is already causing lengthy tailbacks at busy times, is expected to be in place on both southbound and northbound carriageways of the five-mile stretch between Junction 5 at Castle Bromwich and Junction 4 at Coleshill until December 2009.

Speed cameras will be used to enforce the restrictions while the Highways Agency carries out work to turn the hard shoulder into an additional lane and installs equipment in overhead gantries for a variable speed limit system.

The scheme is part of a £6?billion Government plan to tackle motorway congestion. Delays on motorways already cost Birmingham businesses more than £2?billion a year in wasted time.

Birmingham Chamber of Commerce and Industry appealed for “angry” motorists to be patient.

Spokesman John Lamb said the work between Castle Bromwich and Coleshill would mean “short term pain for long term gain”.

Mr Lamb added: “Hard shoulder running proved a success on the M42, but that won’t stop some motorists feeling angry about delays while work is taking place.

“It will be a painful 12 months for many people but we are used to this sort of thing in the West Midlands. There doesn’t seem to be a month that goes by without road works on the M6.”

The Highways Agency has put signs in place warning that the 50mph limit will continue until winter 2009.

A spokeswoman said it was hoped that work would be finished by December next year, but adverse weather conditions could delay the project.

Highways Agency operations director Derek Turner said: “We are looking to keep all three lanes open during construction, especially during busy periods. We will need to close the motorway for short periods but we will do this overnight and will publicise it well in advance.

“When completed, this scheme will bring benefits to road users across the West Midlands and the country as a whole.”

He said average journey times fell by more than a quarter on the northbound carriageway of the M42 during the variable speed experiment and drivers were better able to plan their journey.

Work is also due to begin on installing a variable speed limit system on the M42 between junctions 7 and 9.

A scheme to provide hard shoulder running and variable speed limits between junctions 8 and 10a of the M6 in Staffordshire will get under way early next year, with work unlikely to be completed until spring 2011.