GuideStar UK set itself up with £2.9m from the Treasury in 2003, and has always maintained that it is independent of the American venture.
But Robert Ottenhoff, president and chief executive of GuideStar US, told the US Charity Channel that GuideStar UK was the first progeny of GuideStar International.
Ottenhoff said GuideStar US had two major goals - to continue to expand its domestic database by adding more information from US organisations, and then to bolster it with more data from overseas.
He said: "As part of that effort to expand internationally, the founder of GuideStar, Buzz Schmidt, has gone on to create a new organisation called GuideStar International, on which he is working to create GuideStars in other countries.
"The first of these is GuideStar UK, which will be up and running by the end of the year. We are helping to build its database and we hope that, by next year, we will be able to provide our users with some data generated by GuideStar UK."
Ottenhoff spoke on the radio to promote the revamped US site, GuideStar version 3, which is being launched today. Version 3 offers three tiers of information, one free and two paid-for. The first paid-for level, the 'Select' level, costs $300 (£165) a year; the 'premium' level costs $1,000 (£550) a year.
Eleanor Congreve, operations and fundraising manager for GuideStar UK, said GuideStar UK, GuideStar US and GuideStar International were three separate organisations, and insisted the UK site was independent of GuideStar US.
She said: "Maybe Mr Ottenhoff was referring to the fact that we grew out of GuideStar in the US and that Buzz Schmidt helped us with the feasibility study that led us to get the Treasury grant."
Guidestar UK chief executive Erica Roberts said: "If GuideStar in the US wanted to license data about UK charities, then this would be subject to our standard data licensing terms. To date, however, we have not received an approach from GuideStar in the US.
"That said, we have received interest from a range of organisations in the UK, and are currently in negotiation with five."
Congreve added that the UK site was still on track for a late summer launch to the public.