Britain is being run by a "feral" elite whose members are responsible for a series of crises – from phone hacking to the row over bankers' bonuses – which have scarred the country, a new, non-party group headed by the author Philip Pullman claims.

A 1,000-strong "public jury" should be selected at random to draw up a "public interest first" test to ensure that power is taken away from "remote interest groups" which currently treat the public with contempt, according to the group's declaration. The call for a public jury, which has been signed by 56 academics, writers, trade unionists and politicians from Labour, the Liberal Democrats and the Green party, is published in the Guardian.

Its signatories include Greg Dyke, former director general of the BBC, Caroline Lucas, the only Green MP who is also her party's leader, and the civil liberties campaigner and Labour peer Lady Kennedy. Guardian columnists Polly Toynbee and Madeleine Bunting have also signed the declaration.

Launched by Neal Lawson, a former adviser to Gordon Brown who chairs the left of centre Compass group, the group says that decisive action is needed to wrest power back from a small elite.

"Something is unravelling before our eyes," the group says. "From bankers to media barons, private interests have bankrupted and corrupted the public realm. Power, for so long hidden in the pockets of a cosy elite, has been exposed. Those who wield it have been found wanting - in scruples, in morals and in decency."

The group says that the three crises - MPs' expenses, bankers' bonuses and illegal phone hacking - share common origins. "Politicians, bankers and media moguls ... share a common culture in which greed is good, everyone takes their turn at the trough, and private interest takes precedence over the public good."

In a Guardian article, the authors of the declaration warn of a "feral" elite. Lawson and Andrew Simms, fellow at the New Economics Foundation, write: "With no pressure for higher ethical standards, the new all-powerful elites were like kids left free in the sweetshop, going feral as they lost all self-control and all touch with society."

The group says that 1,000 citizens should be selected at random to sit on a public jury that will propose reforms to banking, politics. The jury, to be funded from the public purse, would examine:

• Media ownership.

• The financial sector's role in the crash.

• MP selections and accountability.

• Policing and public interest.

• How to apply a "public interest first" test more generally to British political and corporate life.

The declaration's main critique of Britain - that power is concentrated in the hands of a small elite - echoes the thinking of Ed Miliband. The Labour leader, who has been praised for shaping the public response to the phone-hacking scandal, recently said that too much power in the media and other industries is concentrated in the hands of too few people.

"The powerful are very good at talking about the responsibilities of the powerless but they aren't very good at looking at their own responsibilities," Miliband told the Times on 23 July as he called for the "big six" energy companies to be broken up. "Labour is the party of the grafters, the people who work hard and do the decent thing but don't feel they get a very fair deal out of society."The declaration is also signed by Lord Wood, an Oxford don who is a senior adviser to Miliband.