The Independent revealed yesterday that Liberty, the civil rights campaign group, was seeking to mount a legal challenge to the operation, claiming it was a "gross invasion of privacy".
Civil libertarians are concerned that motorists waiting to have their vehicles checked for road safety may face questioning from immigration officers, benefits agency inspectors, customs officers and representatives of other government agencies who will be waiting at the roadside as part of the operation.
Liberty spokeswoman Liz Parratt said: "The police have very sweeping and very draconian powers to check vehicle safety and we think that that is right.
"But if those powers are used for purposes other than road safety checks that is when we should start asking questions about whether the police and the other agencies are acting within their powers."
But Bob Nockalls, West Midlands police traffic intelligence officer and national co-ordinator of the operation, said "It's all totally legal.
Of course people have civil rights, and we respect those entirely, but so do the 3,600 people killed on the roads each year and this is a road safety operation."
He said officers would be targeting mainly goods vehicles and obviously unroadworthy private cars.
Vehicles would be taken into check-site areas and while awaiting examination, drivers could be questioned by officers from other agencies acting within their respective powers.
Next month's operation will involve 52 police forces. By casting a wide net, police hope to prevent drivers diverting to miss the check points.
Having representatives from other agencies at the roadside made operational sense, Mr Nockalls said.
For example, Environment Agency officials were better qualified than the police to spot loads of asbestos being ferried to an illegal dump. A previous, smaller, version of the operation carried out in June led to 1,098 vehicles being stopped.
Some 113 people who said they were travelling to work were found to be claiming unemployment or sickness benefit.
That operation is thought to have saved pounds 176,000 in fraudulent benefit claims.
Since the first Operation Mermaid initiative in 1995, 93,000 drivers have been stopped, 673 arrests made and 5,500 dangerous vehicles banned from the road.
One recent exercise in Surrey by the Metropolitan Police resulted in eight arrestsm, of which five were for immigration offences.
Thirty-five of the 404 drivers were reported for benefit fraud, 27 vehicles were banned from the road, and 112 drivers were fined for having invalid tax discs.
A spokeswoman for the Benefits Agency said they could not insist on the people stopped at the road checks taking part in the questioning.
She said: "It has been a successful operation in the past but if people refuse to answer our questions we can't force them."Reuse content