The callers, who usually targeting elderly people, con their way into houses using a variety of stories and then steal property.

Police have described the so-called "distraction burglaries" as a sad and cruel crime.

Detective Constable Jo Chapman, from Lincolnshire Police, said the burglars were often from outside the area.

They purposefully target the most vulnerable members of our community thinking they will less resilient

Dr Amanda Thornton

"These type of offenders come into the county in the morning, they commit offences within the county, then they will be out of the county by the afternoon, offending in another area.

"Historically it has been extremely difficult to catch them, but since joining Operation Liberal we are having some enormous success."

The operation, involving co-ordination with other forces, has resulted in a clear-up rate of nearly 50 per cent and some offenders are now serving prison sentences, she said.

Elderly targeted

Across the East Midlands, nearly 24,000 distraction burglaries are committed a year, Home Office figures show.

Dr Amanda Thornton, a clinical psychologist working on the problem, said elderly people were most often targeted.

"They purposefully target the most vulnerable members of our community thinking they will less resilient, less able to keep them out of their homes."

She said elderly people should keep their doors locked and take precautions when opening them.

'Ruining lives'

"Please use your door chain, ask for identification, check that identification and even ring the company as well.

"That is not being rude, that is being safe."

In Lincolnshire, there has been 69 distraction burglaries this year, with four reported on Monday alone.

Detective Constable Chapman suspected many more such crimes were not reported.

Elderly people were severely affected by the problem, she said.

"It can really ruin their lives... they don't even feel safe in their own homes any more."

"It is a very sad, very cruel crime."