Premier Christian Radio has lost a high court challenge against a ban on an advert seeking information on Christians who feel "marginalised" at work.
The 30-second ad was intended to urge listeners to report their experiences as part of a campaign for "a fairer society".
But on Friday a high court judge ruled that the Radio Advertising Clearance Centre acted lawfully when it banned the ad.
The RACC, which decides whether individual ads meet the radio advertising standards code, said the commercial should not be aired because it was "directed to a political end", and broadcasting it would infringe provisions of the 2003 Communications Act that ban political advertising.
Friday's legal challenge was brought by London Christian Radio, which runs the station, available on medium wave in London and nationally via digital. It was backed by religious magazine publishers Christian Communications Partnership.
At a hearing in March its QC, James Dingemans, described the ad as "about the most inoffensive proposed ad one could hope to get".
Dingemans said if the ad was in breach of the 2003 act, then the relevant sections of the act should be declared "incompatible" with Article 10 of the European convention on human rights, which protects freedom of expression.
Mr Justice Silber, sitting in London, ruled there was no Article 10 infringement and the RACC decision was "rational and lawful".
He declared the ad was political and would contravene the prohibition on political advertising as it was intended to obtain information in a bid "to try to make changes to society".
The judge said the culture secretary, Jeremy Hunt, who has overall responsibility for advertising, had opposed London Christian Radio's application for judicial review.
But the judge stressed that it had not been suggested in any way that Hunt's stance was "anti-Christian", or that his reasoning would not apply to any other religion.
"Indeed nothing in this judgment is meant to preclude advertisements by bodies such as the claimant in, for example, newspapers," Silber said.
The ad was prepared for broadcast in May 2010 after CCP surveys found that more than 60% of active Christians considered members of their faith were becoming increasingly marginalised in the workplace.
Peter Kerridge, chief executive of London Christian Radio and the Premier Media Group, said the case had raised "important issues of fundamental rights to freedom of expression for Christians to report to us their experience in the workplace".