Jowell accuses No 10 of 'meddling' in hunt for BBC boss
David Cameron and George Osborne have been accused of "meddling" in the search for a new head of the BBC Trust.
Former Culture Secretary Tessa Jowell said "interference" from No 10 and the Treasury had made the job an "unappealing prospect" for many.
Interviews to find a successor to Lord Patten, who quit in May, have begun amid uncertainty over the process.
A number of top names, including Lord Coe and Dame Marjorie Scardino, have reportedly ruled themselves out.
The Department of Culture, Media and Sport said the process was "open and transparent".
Senior ministers were said to have thrown their weight behind Lord Coe getting the job but the London 2012 boss reportedly pulled out of the running earlier this week, citing his existing commitments and his wish to focus on becoming president of the International Association of Athletics Federations.
Other leading figures touted for the £110,000 a year job, including former Sony chief Sir Howard Stringer and Baroness Hogg - a former adviser to John Major - are also understood not to be under consideration.
Ms Jowell, who helped set up the BBC Trust when she was culture secretary under Tony Blair, suggested that potential candidates were being put off because of political pressure being applied.
Analysis - David Sillito, BBC Media Correspondent
The reported front-runner Sebastian Coe didn't even join the race.
The rest of the pack, at least the names touted in the press, have one by one revealed they were never in the running and Diane Coyle, the one person to have experience of the job and a declared interest in doing it permanently, didn't even get an interview.
And, for once, the BBC cannot be blamed. It is not their decision.
The only thing you can say for certain is this is a challenging job vacancy for the government. Lord Patten described it as the hardest job he'd done and he's done a few.
The challenges are complex and varied. BBC crises can blow up and blindside even the most cautious; prime ministers, who essentially make the final decision, usually prefer a "friendly face". The candidate needs to be able to grasp and guide the strategic decisions needed to keep a large and unique media organisation going and also accept that in the next two years the job may change radically.
"This meddling by the prime minister and George Osborne is a disgraceful assault on the independence of the BBC," she told the Guardian.
"The behaviour of government ministers has made it an unappealing prospect for very high-calibre potential candidates.
"The BBC Trust needs the best person to chair it who can be confident they can set about the job unencumbered by political meddling. The degree of interference and partisanship seen so far could give no candidate that confidence."
Five candidates are reported to be currently taking part in a initial round of interviews. A preferred candidate will be named and will be questioned by MPs before a final decision is taken.
There is uncertainty over the future of the BBC Trust, with the broadcaster's governance arrangements expected to be overhauled as part of the renewal of its Royal Charter in 2016.
The BBC Trust has been criticised for its handling of the Jimmy Savile sex abuse revelations and executive pay-offs for senior staff at the organisation.
Ministers have said no discussions over charter renewal will take place before the 2015 election, saying this would "tie the hands" of the next government over a range of issues - including the best way to guarantee the BBC's independence and future funding.
But critics have claimed the new chair of the BBC Trust will be effectively acting as a "gravedigger", since the job could change radically or even cease to exist within two years.
"The mishandling of this appointment and delay of any discussion or debate around charter renewal until after the next election worries me," former Culture Secretary Ben Bradshaw told the Guardian.
"It is critical that at this important time for the BBC that the trust has a chairman of calibre who understands the challenges the BBC faces and the political context around charter renewal and the licence fee and it is not someone who makes it look as if the government prejudges the issue."
Lord Patten stood down with immediate effect on health grounds following major heart surgery, with vice-chair Diana Coyle currently serving as acting chair.
In response, a DCMS spokesman said: "The process for appointing the BBC Trust chair is open, transparent and conducted in accordance with the Commissioner for Public Appointments' code of practice.
"A public appointments assessor oversees the process, which includes chairing the interview, to ensure it complies with the code."