Telephone Preference Service 'fails to stop all cold calls'
Only a third of "nuisance" calls are blocked by a service that allows individuals to opt-out of marketing calls, research has found.
The Telephone Preference Service (TPS) runs a register designed to reduce any unsolicited sales calls. Firms can be fined for ignoring the list.
But some rogue companies are flouting the rules, according to regulators.
Some people also unwittingly give consent for calls by ticking a box on online sales forms.
The research, commissioned by Ofcom and the Information Commissioner's Office, found that registering with the TPS blocked 35% of all nuisance calls.
For individuals, registration on the TPS is free and takes 28 days to become effective.
Mobile phone numbers can also be registered, although this will not prevent unsolicited text messages.
It is a legal requirement that all organisations - including charities, voluntary organisations and political parties - do not make such calls to numbers registered on the TPS unless they have the individual's consent to do so.
Sometimes that consent is given in tick boxes "buried in the small print" of online forms, regulators say.
Plans to increase the level of fines levied on firms that make nuisance calls will come into force in October.
Fines of up to 20% of annual turnover could be handed down to firms using information gathered by unlawful unsolicited calls and texts.
"We understand how frustrating it is to still receive some unsolicited sales calls despite being TPS-registered," said Claudio Pollack, of Ofcom.
"That is why we welcome tough enforcement action from the ICO against rogue companies who breach the rules."