BBC current affairs programme Panorama found that despite thousands of complaints to the Information Commissioner each month, there have been no fines on offenders for at least 18 months.
Some 17.5 million numbers are registered with the Telephone Preference Service, which is designed to prevent UK-based companies from making unwanted cold calls.
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Hide AdIndustry rules say telemarketing firms should crosscheck their database to ensure that people who have asked not to be cold called are left in peace.