The UK's immigration rules were changed to grant equality to same-sex couples as an amendment to the Civil Partnership Act, which came into force in December last year. As a result, many Americans with European Union partners have moved to Britain. Lance Lattig, a spokesman for HRW, said: "A large number of same-sex couples have gone to the UK or are thinking of going."

According to the US census there are 40,000 binational same-sex couples in the US, although Mr Lattig believes the number to be several times higher. A Home Office spokesman said: "The act sends a clear message that same-sex couples deserve recognition and respect."

Other countries that allow citizens to sponsor their same-sex partners for immigration purposes include Canada and 12 other European countries - Belgium, Denmark, Finland, France, Germany, Iceland, the Netherlands, Norway, Portugal, Spain, Sweden and Switzerland. On other continents, the list includes Brazil, Israel, South Africa, Australia and New Zealand.

The report says binational couples are "trapped between two ferocious panics sweeping the US". The first concerns gay marriage, which became a divisive issue in the last presidential election. The second is immigration, due to become a key issue in the forthcoming congressional elections after huge pro-immigrant demonstrations and a draconian anti-immigrant bill presented to Congress.