(Reuters) – KPMG mounted an “untruthful” defense when it sought to reduce a fine imposed for its conflict of interest during the sale of British bed manufacturer Silentnight in 2011, Britain’s accounting watchdog said on Wednesday.

KPMG, one of the world’s so-called Big Four accounting firms, was found to have had a conflict of interest when it acted as adviser to both Silentnight and to U.S. private equity company HIG Capital which had sought to buy the British firm.

In August, Financial Reporting Council said it fined KPMG 13 million pounds ($17.7 million), the highest on an accounting firm in a non-audit case, while partner David Costley-Wood was fined 500,000 pounds.

At an independent tribunal in June, KPMG had called for a fine of no more than 5 million pounds.

On Wednesday, the FRC published the tribunal's report on how it determined the level of the fine and the case in general.

“For the first time, the tribunal has held that a respondent advanced an untruthful defense,” the FRC said in a statement.

The FRC had argued that Mr. Costley-Wood, who no longer works at KPMG, assisted with a ‘burning platform’ strategy to drive Silentnight into an insolvency process to help HIG acquire the company without the burden of pension liabilities.

“We do consider that the defense put forward by Mr. Costley-Wood in relation to the burning platform was a construct invented by him to assist in his defense,” the tribunal said.

Mr. Costley-Wood had no comment, his law firm Linklaters said.

The FRC said respondents were entitled to defend themselves but said advancing a defense which a respondent knew to be untruthful risked undermining the regulatory system and compounded the original failings.

KPMG said the report made “difficult reading” and said it accepted the findings and regretted that professional standards expected of its partners were not met in this case.

“We no longer provide insolvency services and we have improved our broader controls and processes significantly since this work was performed in 2010,” KPMG UK CEO Jon Holt said in a statement.

KPMG will reflect on the tribunal's findings carefully to learn lessons, Mr. Holt said.

KPMG is also being investigated by the FRC for its role in auditing collapsed builder Carillion, and any fine from this case would also likely be hefty.