Trading standards officers are urging Bury businesses not to respond to a mail shot that could leave them with a bill of almost Â£2,400.
They advise firms to ignore a letter from a German company headed "Publication of Companies and VAT Registration Numbers in the UK Corporate Portal 2014".
The letter claims that, due to a change to the "EU Economic Package", which requires all companies to provide their VAT registration number on various documents since 2010, the "UK Corporate Portal" is also required to update companies' business data as a "basic entry".
The letter also includes a form for businesses to check and sign to say that their details are correct if they wish their entry to be a "non-basic entry".
However, the small print indicates that a cost of Â£797 a year will be incurred by the business, for a minimum of three years.
Angela Lomax, commercial practices manager with Bury Council's trading standards team, said: "We are concerned that local businesses may believe the letter to be from an official source, which is not the case.
"If businesses sign and return the form to confirm that their details are correct, they could end up tied in to the high costs of an entry in this publication.
"Businesses should be aware that they are not obliged to provide any information to the company and entries do not have to appear on the website referred to in the letter."
For advice, or to report concerns, call the Citizens Advice consumer service on 08454 04 05 06 or go to www.adviceguide.org.uk
We like to keep things running smoothly, please use the button below to report a problem with this page.