Janice Sheridan, 45, and her 70-year-old mother Connie were found stabbed to death in what police described as a "callous and ruthless" attack at their remote home in Upwell, Cambridgeshire.

The reward is understood to be one of the largest offered by a police force, and is the first time Norfolk police have taken such a step.

Detective Superintendent Ian Sturgess, who is heading the inquiry, said: "This is a particularly brutal and callous attack. It is a very, very serious crime in a remote location.

"I believe it is a good way to prompt someone to come forward."

Long-term savings

Norfolk police also believe the reward will save money in the long run.

Detective Inspector Paul Chapman said: "It is a lot of money but in the long run it may well be cost effective if it shortens the investigation and reduces the cost of the inquiry."

He added that so far police have been unable to establish any motive for the crime.

It is thought Miss Sheridan and her mother let their killer into their home and may well have known him or her.

Police also believe the killer locked the front door after committing the crime and then stole the women's keys.

Personal information

Mr Sturgess said the key to solving the crime probably lay in officers finding out more about the private lives of the women, in particular whether Miss Sheridan, a well-known breeder of whippets, was involved in a secret relationship.

The women were well known in the dog world, keeping more than 20 whippets at their home and travelling regularly to dog shows.

Police have been making extensive enquiries among dog breeders to see whether this leads to any clues concerning the killings.

The money for the reward will come out of normal police budgets approved by the Norfolk police authority.