Mr. Mundy emphasized that Clearcast did not actually ban the ad, as it does not have the authority to do so. Rather, the group did not clear the ad for broadcast, a distinction that may have been lost on the nearly 900,000 people who had signed by Wednesday the online petition asking that the ad be allowed to air.

Clearcast rulings must adhere to the U.K. Code of Broadcast Advertising, which reflects the 2003 Communications Act. That law bans political advertising on British TV.

Can political advertising really be banned?

The way free-speech rights are protected and upheld in the United States, where there is an explicit reference in the First Amendment of the Constitution, is different from Britain. (In fact, Britain does not have a single written Constitution, per se, but that’s a different story.)

Television content is governed by the 400 sections of the 2003 Communications Act, an updated version of laws regulating electronic communications, broadcasting services and other media enterprises in Britain.

The prohibition of political advertising was first enacted in the 1950s, when there were only a few holders of broadcast licenses and it was feared they would use their power in the broadcasting world to promote their own agendas.

“The regulation stopped license holders from conveying their views, and denied parties the opportunity to advertise political messages,” Andrew Scott, an associate professor of law at London School of Economics and Political Science, said by telephone.

Political ads have been banned ever since, even during election campaigns, although political parties are each allotted a specified amount of time to present their platforms. The ban on political advertisements applies to nongovernmental organizations and other groups, unless the ad is for a charitable cause.