New European laws banning the sending of unwanted e-mails - spam - come into force on Thursday.
Trying to reject unwanted e-mails can be difficult
The laws make it a criminal offence to send e-mails or text messages unless the recipient has agreed in advance to accept them.
Firms which continue to send junk mail face hefty fines and can even be sued by the recipients.
But it will not free mail boxes of spam as much is sent from outside the EU and escapes the new rules.
The UK Government described the law as a "step in the right direction" in ending the practice of sending unsolicited e-mails.
But new research suggests most top UK websites are already breaking the new rules.
"Companies are either not aware of the legislation, or are ignoring it," said Ian Thomas from WebAbacus.
WebAbacus found 98% of firms do not give enough information about files which track user movements, or provide a simple opt-out option.
The Information Commissioner - the organisation which enforces the regulations - was "very surprised" so many websites were not doing what is required, even though these regulations have been on the horizon for a long time.
"There should be transparency. People should know what is going on with the information collected about them," Phil Jones, assistant information commissioner, told BBC News Online.
Most of the legislation's focus is on the rules that try to curb spam, which now accounts for more than half of all e-mail traffic.
The new laws are the UK's interpretation of the requirements of the EU Directive on Privacy and Electronic Communications that demands member states do more to combat spam.
Fine 'too low'
Unsolicited spam can be sent to companies, but it must have an opt-out clause inside it.
Spam to consumers is banned.
Critics of the UK approach say the laws do not go far enough and that the maximum £5,000 fine is not high enough.
"The whole problem with these laws is that they are geared to spammers being honest and respecting laws," said Steve Linford, founder of anti-spam organisation The Spamhaus Project.
"Of course there are no honest spammers - the whole profession is based on deceit," he said.