London-based Independent International Investment Research says it started using the Gmail name for a web-mail application two years before Google.
Current UK users of Google's service will be unaffected, but a separate trademark dispute forced Google to drop the Gmail name in Germany in May.
There have been talks between the two companies to try and come up with some kind of a settlement; however these talks fell apart after they failed to agree a financial settlement.
From Wednesday morning, new users in the UK signing up with the Google service will be given an e-mail address that ends with "@googlemail.com".
German users with this address report that e-mail sent to their username but ending with "@gmail.com" instead of "@googlemail.com" will still arrive at its destination.