Lorry drivers are preparing to stage an imminent protest on the A9 over plans to introduce average speed cameras without increasing the speed limit for HGVs.

The action in the weeks before Christmas will see drivers stick rigidly to the official 40mph speed limit in what have been styled as "rolling roadblocks".

They will not orchestrate convoys, but drivers say they will form inevitably along the road.

The protest is designed to show the public what it will be like with the cameras, before the road is upgraded to dual carriageway.

Around 150 have agreed to take part. But they will call it off when a start date is announced for a pilot project raising the speed limit for HGVs from 40mph to 50mph on the single carriageway sections of the A9. The Police and the Scottish Government are currently considering such a trial.

It follows the announcement by Transport Minister Keith Brown in July that average speed cameras will be sited along the 136-mile stretch of the road from Dunblane to Inverness to cut down on deaths.

The current limit restricts vehicles above 7.5 tonnes to 40mph, which is 20mph slower than cars on A-class single carriageway roads. However, vans and lorries up to 7.5 tonnes, buses and coaches, motor homes over 3.05 tonnes and cars pulling caravans can all legally travel at 50mph.

Inverness-based HGV driver Conor McKenna, a spokesman for the lorry drivers, wouldn't say when the protest would start.

He said: "All we are asking is to give us a trial at 50mph.

"Car drivers want it. It would improve safety on the A9. But without it there will be more driver frustration which leads to accidents."

A Transport Scotland spokesman said: "As stated previously, the A9 Safety Group, Ministers and Police Scotland have not ruled out the introduction of 50mph speed limits for HGVs in future if evidence supports this.

"Further investigation work into the effects of increasing the HGV speed limit has been undertaken."