Motorists face months of delays when a £44.4m project to replace an ageing viaduct begins on Wednesday.

The Highways Agency wants to rebuild the Wolvercote Viaduct on the A34 near Oxford over a two-year period.

It has warned drivers delays are expected at peak times when a 40mph limit on the A34 and 30mph on a stretch of the A40 is enforced.

The viaduct, built in 1960-61, needs to be replaced for safety and to meet current design standards.

The work on the 1km stretch of road will be completed in several phases, with the first phase expected to end in spring 2009.

There will be two narrower lanes for 500m in both directions on the A34 in the day, and some road closures overnight for the rest of this year and the start of next year. A 40mph speed limit will be enforced by average speed safety cameras.

On the A40, which runs under the viaduct, there will be a narrower lane running in each direction with a 30mph speed limit.

There will be a temporary diversion to a nearby footpath and cycleway.

Drivers will be diverted on to a temporary southbound bridge deck while work goes on.

Roads minister Tom Harris said: "It's good news for drivers that the Highways Agency will be able to completely replace this viaduct while keeping the road open to vehicles.

"The A34 is a key route between the Midlands and the south coast and while this improvement work is important, it's essential that any delays to drivers are limited."

The Wolvercote Viaduct carries the A34 dual carriageway over the River Thames flood plain, the Oxford to Birmingham main line railway, the Oxford Canal and the A40 Oxford to Cheltenham Road.

Rail passengers travelling on the Oxford to Birmingham line will not be affected.

A temporary viaduct will be built along the southbound carriageway of the existing viaduct, nearer to Lower Wolvercote and Oxford while construction takes place. This will be demolished once the replacement is built.

It is hoped the new viaduct will be up and running in 2010.

A public exhibition by the Highways Agency will take place in Wolvercote Young People's Club in St Peter's Road on Friday, May 30, between 3pm and 9pm and Saturday, May 31, between 10am and 4pm when residents can ask questions.