The BBC and ITV face a legal challenge after asking a Christian party to make changes to its political broadcast.
Christian Choice says both broadcasters have effectively "censored" its film which criticised a Muslim group over its plans for a 12,000-capacity mosque.
Its legal team are seeking a judicial review of the decision and have filed papers at the High Court.
ITV and the BBC say they have to ensure content is legally safe and meets broadcasting requirements.
Alan Craig, the Christian Choice's London mayoral hopeful, says elections are about "open discourse and a free exchange of views".
The Christian Choice is an electoral alliance between two political parties - the Christian Peoples Alliance and the Christian Party.
Mr Craig opposes plans to build a large mosque in east London and the group behind it, Tablighi Jamaat.
He says the BBC and ITV are guilty of "political censorship" in asking for changes to the party election broadcast (PEB) - the full version of which is running on YouTube - in the way he describes the group.
He also argues the action breaches his rights under the European Convention on Human Rights - which guarantees the right to freedom of expression.
"Nationally and locally, leading Muslims share my objections to the mega-mosque, yet two national broadcasters have attempted to close down the democratic debate, through misplaced concern," he said.
An ITV spokesman said the editorial content of PEBs was a matter for political parties.
"But each broadcaster is obliged to ensure they meet legal and broadcasting regulatory requirements," he said.
"That is the only context in which ITV will occasionally ask a political party to remove or change an aspect of a broadcast."
A BBC spokeswoman said a number of issues had been raised with the party over the broadcast, including technical problems and a minor legal concern.
"We offered advice as we often do to help parties achieve appropriate broadcast standards. They subsequently submitted an amended version which we were happy to transmit, " she said.
"Each broadcaster has an obligation to ensure content is legally safe and meets broadcasting requirements."