A third of the UK's top companies are not complying with the European Union's (EU) regulations on unsolicited emails, or spam, a report has alleged.
Despite filter software, spam emails keep on getting through
According to data management firm CDMS, firms are not asking people if they want to receive the emails.
Instead, when people sign up for promotional offers or competitions they often only get the option of opting out, or unclicking a pre-ticked box.
However, the body charged with policing the law said its powers are limited.
The Information Commissioner's Office - an independent body appointed by the Crown - said that while it has the power to fine transgressors up to £5,000 it often proves impossible to track them down.
Many companies that send spam emails operate outside of the UK, the ICO said.
Those that are registered in the UK usually stop sending emails once they have been contacted and warned by the ICO, a spokeswoman explained.
EU laws have made it illegal to send emails unless they are asked for
She said the ICO has been in talks with the Department of Trade and Industry over what can be done "to strengthen our enforcement powers to deal with the irresponsible minority".
The majority of UK firms have taken steps to stop spamming, data company CDMS said.
"Experienced marketing directors have recognised and accommodated this restriction," CDMS said in a statement.
"We would exhort those who have not yet paid full attention to this issue to do so with all speed, before enforcement test cases start to be launched and before consumer lobby groups blacklist them."
CDMS said it examined the compliance of 200 top UK companies across sectors including banking, insurance, publishing, broadcasting, retail and telecommunications.
Authorities are stepping up their efforts to limit the amount of unsolicited emails flooding mailboxes and earlier this year a former AOL employee was sentenced to 15 months in prison for selling members' details to spammers.