The data protection watchdog is to contact more than 1,000 firms that buy and sell people's personal details to ensure they are acting lawfully.
The Information Commissioner's Office (ICO) says it will this week write letters to the firms, which are all believed to play some role in the compiling and trading of people's names and numbers used by cold callers.
The firms will be required to set out how they comply with the law, including what data they share, how they get people's consent to share their data, as well as a list of all the companies they have worked with in the last six months.
If companies do not respond to the letter, the ICO says it will take action to require the information be provided. The watchdog has the power to issue 'information notices', which legally oblige an organisation to provide it with information, with the threat of court action if they do not.
Separately the ICO will issue three fines to firms this week totalling £250,000, meaning it will have issued £1 million worth of penalties in this sector over the past four months.
The cold calling sector generates 180,000 complaints a year from people angered by nuisance calls. See MoneySavingExpert.com's 'No cold callers' guide to stop spam calls, texts and mail.