British actors who have worked in Hollywood, including Ewan McGregor, Kate Winslet and Hugh Grant, are members of SAG.
The Screen Actors Guild (SAG) has issued guidelines to its non-US members, warning them not to work on productions shot outside the US which have American backing.
The union is seeking to prevent film studios simply exporting productions to other countries, such as Canada and the UK, while the strike takes place.
Writers and actors want a bigger share of the revenue earned by sell-on rights - such as for video, DVD, cable and internet - as well as a bigger say in the creative process.
Strike talks
The contracts of the 14,000 members of the Writers' Guild of America (WGA) expire on 1 May and the deal for 135,000 members of the Screen Actors' Guild (SGA) is up for renewal two months later.
Talks between producers and writers to avert a strike broke down earlier this week, raising fears that Hollywood production could grind to a halt.
Many pundits believe strike action is now a question of when and not if.
John Drinnan, from Screen International, told the Financial Times that the strike would affect Britain "very badly".
He said: "The majority of British films with a budget above £10m - the ones that potential to be box office hits - have American money and have rights sold to a US company."
British actors who have worked in Hollywood, including Ewan McGregor, Kate Winslet and Hugh Grant, are members of SAG.
The guidelines mean the stars would not be allowed to work on films in Britain, if they are US-financed.
British actors who are not SAG members will also find it difficult to work on US productions in the UK.
A spokesman for Equity told BBC News Online: "Equity members who are also members of SAG will not be able to work on productions in the UK that have any American involvement, such as financing and pre-sales for the American market."
"The strike will have an impact on Britain," he added.
UK support
The union is also advising its members, even if they are not SAG members, not to accept work on so-called runaway productions.
A runaway production is a film shot in the UK for the sole purpose of undermining the SAG action.
"We are looking to SAG to advise us on what is a runaway production," said the Equity spokesman.
He added: "So far we have not been advised that there are any runaway productions."
He said the British union supported SAG's attempts to get a new contract.
He added: "We believe that is it is achievable without a dispute."