British broadcaster Channel 4 represented Prime Minister Boris Johnson with a block of melting ice in a prime-time election debate on the environment on Thursday, prompting his Conservative Party to complain this broke impartiality rules.

The commercially funded public-service broadcaster invited leaders of all Britain’s main political parties to take part in the debate before December 12’s election, but both Johnson and the leader of the Brexit Party, Nigel Farage, declined to attend.

The Conservative Party offered former environment minister Michael Gove as a substitute, but the broadcaster said the debate was only intended for party leaders, and that the other political parties would not agree to change the terms.

“This effectively seeks to deprive the Conservative Party of any representation and attendance,” the Conservatives wrote in a letter of complaint to broadcast regulator Ofcom.

British television broadcasters are required to be politically impartial, and face extra balance requirements during election periods. Ofcom can fine broadcasters that do not comply, and as a last resort can cancel a broadcaster’s licence.

The Conservatives said Thursday’s disagreement was “part of a wider pattern of bias by Channel 4 in recent months”.

The broadcaster’s head of news and current affairs, Dorothy Byrne, described Johnson as “a known liar” in a major industry speech in August.

On Thursday, the editor of Channel 4’s main news programme, Ben de Pear, likened the Conservatives’ approach to the media to that of US President Donald Trump.

Conservative Party chairman James Cleverly said Britain’s election was parliamentary, not presidential, making it unreasonable for Channel 4 to require Johnson’s appearance as a condition of the Conservatives taking part in the debate.

“Put your leader Boris Johnson alongside the other leaders and stop playing games. Don’t refuse and then threaten our licence – it’s a slippery slope,” de Pear said in reply.

News website Buzzfeed quoted an unnamed Conservative Party source as saying the party would call for a review into Channel 4’s operating licence, which is due for renewal in 2024, if Johnson returns to power after December 12’s election.

Channel 4 is not the only broadcaster to struggle to book Johnson, whose Conservative Party is leading the opposition Labour Party in opinion polls and keen to avoid any gaffes.

Johnson has so far declined to take part in a half-hour sit-down with BBC interviewer Andrew Neil, who gave tough scrutiny to Labour leader Jeremy Corbyn on Tuesday.