The government is powerless to intervene in a four-year inquiry into the collapse of Rover, which has cost almost £15 million and has no end in sight.

Ministers and officials in the Department for Business and Enterprise fear any interference could lead to claims that it has not been impartial.

Officials from the Department for Business, Enterprise and Regulatory Reform held ten meetings in the past year with accountants conducting the inquiry, to “discuss progress” as their work drags on. But they will not go further and ask the inspectors to hurry up, as this could provide grounds for former Rover directors to challenge the findings when they are eventually published.

Ministers appointed forensic accountancy firm BDO Stoy Hayward to comb Rover’s accounts following the collapse of the Birmingham carmaker in 2005.

The inquiry is led by Gervase MacGregor, the head of the BDO Stoy Hayward team, and barrister Guy Newey QC.

But there has been growing anger that the investigation, originally expected to last 18 months, has still not published its findings, with ministers privately admitting they are growing impatient.

One question the inquiry is expected to answer is whether the four Rover directors, who bought the carmaker for £10 in 2000, were partly responsible for its downfall. John Towers, Peter Beale, Nick Stephenson and John Edwards were widely criticised for taking millions out of the company before it collapsed, leaving 6,500 workers without jobs.

But the so-called Phoenix Four, who called their consortium Phoenix Venture Holdings, could challenge the inquiry’s findings if there were suggestions of government interference. The total cost of the inquiry as of February 28 was £14,806,505, including £12,167,538 in fees, £447,613 in disbursements and £2,191,354 in VAT.

A Department of Business spokesman said: “Officials regularly discuss the likely timescales of specific parts of the inspection process and the timetable of the inspection as a whole with the inspectors. However, no firm date has been set for its conclusion. The Department does not consider setting such a date to be practical or realistic.”

MP Richard Burden (Lab Northfield), whose constituency includes the former Rover plant at Longbridge, called on the inspectors to end the investigation. He said: “Nobody wants the inquiry to be rushed but after four years it is hard to believe they haven’t had enough time. Reports into the Iraq war haven’t taken as long as this. The Stern report on the environment, looking at the future of the planet, was completed more quickly.”

The wait has left up to £16 million in a fund pledged to former Rover workers marooned in a bank account, scuppering potential pay-outs of four-figure sums for ex-employees. Rover Trust Fund member Carl Chinn said the delay was “ludicrous”. He said: “Many of these workers are still struggling. They are in temporary, low-paid and insecure work and they need the money now.”