The founder of the Transparency Task Force (TTF) has demanded a public apology from the UK asset management trade body over a “misleading and offensive” press release about cost transparency.

In an email to Investment Association (IA) chief executive Chris Cummings – also copied to the chief of the UK financial regulator and prime minister Theresa May – Andy Agathangelou said the trade body should “take action and admit that it has been completely wrong about hidden fees for a long, long time”.

The dispute dates back to a press release from the IA in 2016, which argued that accusations of hidden fees in asset management were unfounded. The release referred to hidden fees as “the Loch Ness Monster of investments”.

Agathangelou, founding chair of the TTF, which campaigns for cost transparency, wrote to Cummings after the pair appeared on a BBC radio programme about investment fund transparency last night.

Agathangelou wrote: “In light of the admission you made in yesterday’s programme and in light of the fact that MiFID II has now completely disproven the assertion that there is no evidence of hidden fees… I believe that now is the right time for the Investment Association to take action and admit that it has been completely wrong about hidden fees for a long, long time.”

According to research published earlier this week by Scottish consultancy The Lang Cat, newly disclosed transaction costs under MiFID II showed fund fees for some of the most popular UK funds were significantly higher than previously indicated. The difference ranged from a few basis points to 85%; the sample size was 20 funds.

In the BBC radio programme, the IA chief executive said the language used by the industry association in the 2016 press release was “regrettable”, but added: “That decision was taken before I arrived and there is nothing I can do to change that.”

Cummings later agreed that “100% of [IA] members are 100% behind full transparency of costs and charges”.

Speaking to IPE shortly after the Loch Ness Monster press release was published, the IA’s then-chair Helena Morrissey said the association had “inadvertently but understandably upset people”.

Responding today to Agathangelou’s message, the IA said: “The Investment Association is now under new leadership and is wholly focused on delivering on the issues that consumers and our industry care about, such as providing full transparency on costs and charges.

“A successful example of our work in this area to date includes developing a new cost disclosure template with the Local Government Pension Scheme Advisory Board as part its Code of Transparency, which was successfully implemented last year.”

“We are also working closely and productively with transparency campaigner Chris Sier as part of his role as chair of the FCA’s independent working group, to design a new template on cost disclosure which will be rolled out later this year. The group has made excellent progress on this initiative so far and the results will be consulted upon soon.”