Interel's boss told PRWeek the Association of British Bookmakers (ABB) was "throwing dirt" in an attempt to "defend the indefensible" - but the association accused the parliamentary group of "flimsy research and secrecy".
In January, the Fixed Odds Betting Terminals All-Party Parliamentary Group (FOBT APPG) released a report calling for tougher laws on the "highly addictive" machines. The Association of British Bookmakers complained shortly after to the parliamentary watchdog, saying the group had "not been transparent about their nature, membership and funding".
The commissioner's findings, published on Wednesday, concluded that the group had breached four paragraphs of the rules covering the APPGs, including a failure to declare on the group's website that Interel's provision of secretariat services to the group was done on a pro-bono basis.
The report also found that the group failed to identify in the report which of its members were present at certain meetings, and correctly declare its status, when it sent an interim version of its report to the Department of Culture, Media and Sport, when that department was running an inquiry into fixed-odds machines.
The commissioner said the breaches were "all at the less serious end of the spectrum" and that concluded that the group "had not sought to mislead". The group's chair, the Labour MP Carolyn Harris, has apologised for and rectified the breaches.
ABB CEO Malcolm George told PRWeek: "This is a damning verdict by the Parliamentary Commissioner for Standards against the All-Party Group and its secretariat, Interel. The secrecy with which Interel acted in failing to declare its status as donor in a report that it helped write is astonishing."
The ABB said that Interel works for "direct rivals" of bookmakers who operate such machines. The APPC register lists Hippodrome Casino and gaming tech firm Novomatic as clients.
George McGregor, managing partner of Interel, said that with parliament dissolved, the group itself could not respond to the ABB boss' comments.
However, he argued that the report recognised that the breaches were "minor" and "inadvertent" - and pointed out that they had been rectified quickly.
McGregor said: "The commissioner also acknowledges that there is ambiguity about some aspects of the parliamentary rules and that there was no intention by the group to mislead.
"We are cynical about the reasons for the ABB choosing to make this complaint. It was the classic PR manoeuvre. When you know a bad story is coming throw as much dirt as you can to change the story.
"Their problem is they are defending the indefensible: dangerous and addictive fixed-odds betting terminals, known as the 'crack cocaine' of gambling, which are causing untold harm to communities and individuals. We will continue to help our clients tell that story."