Five bikers and bike passengers died in three days in July

The operation, which is taking place through December, is part of efforts to cut the number of bikers killed.

Officers say that one in five motorcyclists who are discovered speeding at more than 100mph (161km/h) have false or illegible plates.

In July, there were five bike-related deaths in less than three days in Devon and Cornwall.

Motorcyclists have to have these plates made up illegally Bill Skedgell, DVLA

Police said plate tampering included changing the size or the position of a plate, or using different typefaces which were more difficult to photograph.

Chief Inspector Ian Aspinall, Head of Road Policing, said: "A number of motorcyclists make concerted efforts to avoid speed detection by interfering with the display of their number plate.

"It may be that in doing this they feel that they can ride as fast as they like without fear of detection.

"We will be targeting individuals who are blatantly flouting the law in this way."

New laws have also been introduced to clamp down on manufacturers and shops that make and sell reduced size plates knowing they are for a road-going bike.

'Show' plate

Bill Skedgell from the Driver and Vehicle Licensing Agency (DVLA) said: "Motorcyclists have to have these plates made up specially.

"Many reputable manufacturers will not perform this task because of new legislation. Some are being less scrupulous and accepting a disclaimer from the owner that the number plate is not for use on the road or is a 'show' plate for display only, but this is not acceptable.

"The DVLA is keen to ensure manufacturers understand their obligations and those who transgress will be prosecuted and can face having their registration licence to supply plates suspended."

Nationally, motorcyclists make up just 1% of traffic on roads but account for 14% of deaths and serious injuries in crashes on the roads.

In Cornwall, 25% of all fatal road accidents in the county involve motorbikes.