One of the UK's largest accounting firms, Deloitte, failed to manage conflicts of interest when advising MG Rover Group and the "Phoenix Four" directors who bought the British carmaker before it collapsed.

The Financial Reporting Council (FRC), the body that regulates accountants, said last year that Deloitte and its employee Maghsoud Einollahi did not properly manage the dual roles of acting as corporate advisers to companies involved with MG Rover and the four businessmen, while the accounting firm was simultaneously auditing MG Rover. The firm's challenge to that decision was thrown out in a hearing on Monday at the international dispute resolution centre.

Paul George, FRC executive director for conduct, said: "The outcome of this tribunal sends a strong clear reminder to all accountants and accountancy firms that they have a responsibility to act in the public interest in the work they undertake."

The ruling is the latest outcome in the sorry tale of MG Rover, which was bought in 2000 for £10 by the Phoenix Four of John Towers, Nick Stephenson, John Edwards and Peter Beale, who then paid themselves and managing director Kevin Howe a total of £42m before the company went into administration with debts of £1.4bn in 2005.

Around 6,000 MG Rover workers who lost their jobs in the debacle received next to nothing in compensation. The quartet were all disqualified from working as company directors in Britain in 2011 for a total of 19 years.

Deloitte argued that Monday's decision could have wider implications and force all accountants to examine what advice they can give. It said it would consult with business and accounting bodies on whether to mount a further appeal on some points.

A spokesperson for the firm said: "We are surprised and very disappointed with the outcome of the tribunal and disagree with its main conclusions. Deloitte's advice, which itself was not criticised, helped to generate over £650m of value for the MG Rover Group, keeping the company alive for five years longer than might have been the case and securing 5,000 jobs in the West Midlands during this period. We take our client and public interest responsibilities extremely seriously and are proud of the value we helped create for the MG Rover Group".

The £650m of value Delotte argues it helped generate predominantly consisted of cash balances already in the company, MG Rover's existing stock of cars, the carmaker's parts business which was later sold for around £100m, plus an interest free loan from BMW to Techtronic, the Phoenix Four's holding company.

The tribunal is now hearing what sanctions the FRC wants to impose. It has powers to impose unlimited fines and suspensions.