Ian Leggett, director of coalition member People and Planet, issues the warning in this week's Hot Issue.
He says the media regulator's decision has "implications beyond MPH.
If we are prevented from using film advertising on the grounds of politics, it will undermine our effectiveness."
His comments come after Ofcom ruled MPH's campaign was political because it sought "to achieve important changes to the policies of the UK Government".
Stop Climate Chaos placed a full-page colour ad in The Times last week, carrying the headline "Global Warning". The advert urges readers to lobby the Prime Minister to act now before it's too late.
But the Advertising Standards Authority, which adjudicates on newspaper ads, has different guidelines from Ofcom. It said the ad had so far drawn no complaints and was unlikely to constitute a breach.
- See Hot Issue, page 23.