The DUP claims that the BBC has "serious questions to answer" over allegations made in a Spotlight programme about the party's spending during the Brexit referendum campaign.
The Electoral Commission announced yesterday that it will not be launching an investigation into claims made in the documentary, Brexit, Dark Money And The DUP.
The Spotlight show asked questions about whether the DUP incurred joint spending with other EU referendum campaigners but did not declare it under a common plan.
Under Electoral Commission rules, groups are not allowed to work together in order to get around funding limits.
The DUP denied wrongdoing and always insisted it acted within electoral law.
Following the Commission's announcement, East Londonderry MP Gregory Campbell claimed that the BBC programme was "heavy with innuendo but light on facts". He said: "The Electoral Commission has issued a statement after an issue was raised about 'Brexit spending' by the DUP following a so called 'investigative journalism' programme by the BBC NI Spotlight team.
"The Commission said they 'do not have grounds' to open an investigation.
"This latest embarrassment for the BBC comes after a series of official complaints levelled against the BBC in Northern Ireland at the highest level in London about their political bias."
Mr Campbell also called on the BBC to answer a series of questions about the broadcast.
He said: "Will there be a detailed breakdown of the amount of public money spent on producing the programme?
"What countries were visited in the course of making this programme, how long was spent there and by whom?"
The MP also challenged the BBC to state whether they had paid for an interview with him which was included in the show and "was not authorised by me or provided by me for the programme".
After the documentary aired in June, the Electoral Commission requested further evidence from BBC Northern Ireland but were told there was no other significant information other than what was in the programme.
The Commission considered whether other sources were available as evidence, but concluded it did not have grounds to open an investigation.
A statement noted that it "continues to be prohibited by legislation from disclosing any information concerning donations to Northern Ireland recipients made prior to 1 July 2017".
"We continue to urge the UK Government to bring forward legislation that will enable us to publish information on donations from January 2014," it said.
Alliance MLA Stephen Farry claimed the Commission's findings did not amount to an exoneration of the DUP.
"The serious questions regarding the funding and spending of the Leave Campaign in the Brexit referendum and the involvement of the DUP persist.
"Satisfactory explanations around a number of issues and situations remain," commented Mr Farry.
"As the complexities and difficulties of the Brexit process become ever clear, with continued unanswered allegations and unexplained aspects of the Brexit referendum, the legitimacy of the 2016 vote is increasingly called into question."
Mr Farry said the decision "highlights the lack of robustness of the law, practice and accountability regarding the regulation of election campaigns and referendums".
He said the Commission appeared unable to do more within the existing legal framework.
A BBC spokesperson said: "This Spotlight investigation raised important issues of public interest and was made in accordance with the BBC's editorial guidelines, including those relating to accuracy and independence.
"It was carefully and extensively researched and put significant evidence into the public domain."