The Lib Dems have made a formal complaint after ITV said its head-to-head election debate would only include Boris Johnson and Jeremy Corbyn.
Party president Sal Brinton said leader Jo Swinson should appear alongside the Tory prime minister and the Labour leader in the 19 November debate.
Baroness Brinton said Ms Swinson's exclusion risks "misrepresenting the current political reality".
ITV said it intends to offer viewers balanced election coverage.
In a letter to ITV's chief executive, Dame Caroline McCall, Baroness Brinton wrote "voters of this country deserve to hear from a Remainer on the debate stage, not just from the two men who want to deliver Brexit".
The Lib Dems have pledged to cancel Brexit if they win the election as a majority government.
ITV, which announced the head-to-head election debate on Friday, said it would also hold a "multi-party debate" before the 12 December poll. The Conservatives, Labour, Lib Dems, SNP, Brexit Party and Plaid Cymru will take part, represented by either their leader or "another senior figure", it said.
'No reasonable justification'
The head-to-head debate will be hosted by news presenter Julie Etchingham and take place on Tuesday 19 November.
After the main event, ITV said it would hold a live interview-based programme to allow other parties to comment on the debate.
Broadcasting rules in place during the official election campaign period require producers to ensure "due weight" is given to coverage of political parties and candidates.
In her letter, Baroness Brinton said: "There is no reasonable justification for excluding Liberal Democrats from the debate. Liberal Democrats are the strongest national party of Remain.
"We secured more votes than both Labour and the Conservatives in the European elections earlier this year and have enjoyed fantastic local and by-election successes across the country."
An ITV spokesman said: "ITV intends to offer viewers comprehensive and fairly balanced General Election coverage.
"This involves a wide range of programming, including a live debate programme in which seven party leaders are invited to take part, as well as a live debate between the Labour and Conservative leaders."
Political leaders' TV debates have featured in the last three general elections in 2010, 2015 and 2017.