The Liberal Democrats have accused the BBC of backtracking on a deal about the final round of TV interviews with party leaders, and misleading the country by reporting Nick Clegg chose to opt out of last week’s debate.

Lord Ashdown, the party’s campaign chief, has written to the BBC director general, Tony Hall, saying he will make a formal complaint to the BBC Trust unless the broadcaster immediately corrects suggestions that the Lib Dems opted out of the debates attended by the five opposition parties.

“Nick Clegg did not chose to ‘sit this out’; he was excluded from the debate by the BBC. At no point were the Liberal Democrats offered a choice that involved allowing Nick Clegg to take part. Your executives will confirm that we consistently told the BBC that he wished to be included in the debate,” Ashdown says in the letter.

“You will understand how damaging it is to suggest that Nick Clegg voluntarily chose not to participate when that is not the case and further to equate our position with that of David Cameron, who did refuse to participate and who has consistently sought to avoid debating other leaders.”

Lib Dem sources say Clegg was not invited because the BBC knew Cameron would not attend, and the broadcaster decided it dare not empty chair the prime minister, so came up with the idea that neither government party attend.

Ashdown has also registered dismay that the BBC had reneged on an agreement about subsequent debates.

Clegg did not press the issue of his presence at last week’s debate partly because of an assurance that only the Lib Dem leader, and Cameron and Ed Miliband would appear in a final set of successive interviews each lasting half an hour as part of a single 90-minute programme.

But since then the BBC has offered Nigel Farage, the Ukip leader, a further 30-minute slot in England, as well as 30-minute interviews for Leanne Wood, the Plaid Cymru leader in Wales, and for the Scottish Nationalist leader, Nicola Sturgeon, in Scotland.

Clegg believes these extra programmes undermine the deal whereby he backed down over the 16 April debate because he alone of the party leaders unlikely to become prime minister would be allowed in the final interviews.

Ashdown writes: “We are currently in discussions with BBC executives to understand why this has happened and how the BBC will guarantee that any new arrangements ensures that the Liberal Democrats are not further disadvantaged compared to those parties that were not excluded from the debate on April 16th. I hope you will ensure that this happens.

“In view of the seriousness of the misrepresentation of our position on the BBC News at Ten and elsewhere, I would be grateful for rapid confirmation that an apology and clarification will be forthcoming. In the absence of such confirmation, we will need to consider a formal complaint to the BBC Trust.”

The complaint underlines the battle the Lib Dems have to ensure Clegg gets coverage in the election and is not squeezed by the main two parties.