The lanes, opened yesteday, mean there are now four lanes on sections of both carriageways between Junction 11 and 12, Fareham and Portsmouth.

But motorists initially seemed confused by the new merging systems which operate where traffic comesg on to the motorway, and where the fourth lane disappears.

Kelly Fryer, 20, of Devonshire Way, Fareham, said: 'I drove from Fareham to Portsmouth. When you get on the motorway, the left-hand lane of the slip road now carries on to become the new slow lane, and then those driving in the right-hand lane on the slip road have to merge into the second lane.

'Loads of people seemed to be getting confused by it. There were brake lights everywhere, although you do get that kind of merging system elsewhere.

'I guess it's just a case of people getting used to the new layout.'

The £18m widening scheme is designed to give more chances to overtake traffic going uphill slowly, ease delays, and improve safety through separating slower and faster vehicles.

The project was supposed to finish in October.

Stephen Coe, project manager for the Highways Agency, said: 'We have been able to open the climbing lanes two months early, so keeping delays to a minimum.

'I would like to thank drivers for keeping to the speed limits through the works and thereby keeping traffic flowing well even at peak times.'