The Royal Mail owns and maintains the database of postcodes
Websites that help people find jobs or hospitals have been hit by legal action threatened by the Royal Mail.
The threat was issued against the company supplying them, and many other sites, with postcode data.
Royal Mail said the legal action was threatened to stop "unauthorised access" to the postcode data.
Ernestmarples.com, which supplied the address data, said it did not have the resources to fight a legal battle so has turned off its feed.
Sites affected by the withdrawal of data include Job Centre Pro Plus, HealthWare (locates nearby pharmacies and hospitals), Planning alerts.com (monitors planning applications), Straight Choice (finds out who sent political leaflets) and many more.
Commenting on its action the Royal Mail said: "We have not asked anyone to close down a website.
"We have simply asked a third party to stop allowing unauthorised access to Royal Mail data, in contravention of our intellectual property rights," it added in a statement.
The third party was Ernestmarples.com which supplied postcode data to the sites. Typically those wanting to use the regularly updated list of 1.7 million postcodes pay the Royal Mail for access.
Many of the sites hit by the action are volunteer efforts that would be unable to afford the licence fees - about £4000 per annum for unlimited access.
Harry Metcalfe, co-founder of Ernestmarples.com, declined to say how it got access to the postcode data.
"There are certain legal questions around the specifics of what we are doing," Mr Metcalfe told BBC News. "The advice we have received so far is that allowing the service to continue operating while we attempt to resolve this issue would be unwise."
In the interim, the company is searching for alternative free sources of postcode data even though those are known to be less accurate and not as regularly updated.
Mr Metcalfe said Ernestmarples.com would talk to the Royal Mail about getting access to the full postcode data.
Ernestmarples.com is named after the post master general who oversaw the introduction of postcodes in 1959.
Jim Killock, head of the Open Rights Group which campaigns on digital issues, said the action showed why the UK needed a more open approach to official data.
"It is easy to see that large numbers of small business ideas and not for profit services are being blocked by these licence fees," he said. "It is in effect a tax on innovation."