The imposing, elliptical table in the Cabinet Room was originally ordered by Harold Macmillan so that he could see the faces of all his assembled ministers during meetings. As such, it’s a useful metaphor for the collective nature of the cabinet itself – the body that traditionalists see as the decision-making bedrock of the UK’s informal system of government.

Though complaints about the undermining of the primacy of cabinet are hardly new – Margaret Thatcher’s style of leadership prompted a noisy chorus of such voices – they gained new force this summer when all four surviving ex-cabinet secretaries, giving evidence to the House of Lords constitution committee, questioned the ever-growing profile and power of prime ministers. Lord Armstrong of Ilminster said Tony Blair’s style had “conflicted with… the fundamental position of the relationship between the prime minister and departmental ministers”. Lord Butler of Brockwell complained that both Blair and Gordon Brown were “not very much disposed to using cabinet government”.

New Labour’s approach has infuriated traditionalists, and senior Tories have talked of ending the era of “sofa government” – the pejorative expression applied to Blair’s style of hatching informal deals in small groups of ministers and officials. George Jones, Emeritus professor of government at the London School of Economics (LSE), made a written submission to the peers and is highly critical of Blair’s “presidential” style – a product, he says, of his unfamiliarity with government.

“He’d always been in opposition, he’d never been a junior minister or a parliamentary private secretary and didn’t know how government worked,” Jones says. “All he knew was how he’d transformed the Labour Party – in a very top-down way.” Both Blair and Brown have a reputation for preferring small coteries of advisers over cabinet colleagues, and Jones doubts that Brown has changed since entering Number 10.

However, one of the special advisers most closely associated with ‘sofa government’, Tony Blair’s ex-adviser Jonathan Powell, doesn’t lament the demise of highly-strung cabinet decision-making. The former chief of staff bluntly informed the peers that “the cabinet is not the right body in which to attempt to make difficult decisions”. In his written submission, he insisted that “since at least the 1970s” the cabinet has ratified decisions rather than made them. Sir Richard Mottram – who held a clutch of jobs in Whitehall – was similarly cautious about nostalgia for a 1970s “golden era”: it was, he told the committee, a “time of serious failure for the UK as a country and for its system of government”.

Peter Riddell, a longtime Whitehall-watcher for the Times and now a fellow of the Institute for Government, says that while Blair completely neutered the cabinet in his early years, as he gained experience in government he came to see its formality as “useful political insurance on big decisions like Iraq and the Euro”. But Blair wasn’t the only one sidelining cabinet: when Brown was chancellor, says Riddell, “he kept economic decisions to himself – not discussing them with Blair, let alone the cabinet”.

As prime minister, Brown pledged a less presidential style in his opening days, and upped the profile of cabinet meetings by staging them outside London for the first time in decades – but the testimony of one ex-minister suggests that his approach may not have differed quite as much as advertised. Upon leaving her job as Europe minister – which entitled her to attend cabinet meetings – Caroline Flint promptly accused Brown of operating a “two-tier” system, with a trusted few in his inner circle, and the rest of the cabinet kept at a distance.

Beyond conforming to constitutional niceties, though, are there good reasons for prime ministers to subject decisions to rigorous discussion in cabinet? Sir Robin Mountfield, who formerly held permanent secretary rank at the Cabinet Office and ran the Office of Public Service for three years, insists the cabinet is indispensable to robust scrutiny and debate. “It does need to have some real substance, for two reasons: to maintain the collective responsibility of the whole government; and to ensure there is a live forum for internal challenge on the really big issues,” Mountfield says. “The potential for challenge seems to me to be at risk if the real decisions are taken in small informal groups of ministers picked for the occasion by the prime minister.”

Those decisions not taken in the prime minister’s inner circle may often be taken in smaller, policy-specific cabinet committees, which are used increasingly and offer something of a halfway house between the formality of full cabinet and private, cosy deal-making. But sceptics argue that these forums can still be swayed by the dominance of senior figures – and point out ominously that Lord Mandelson, Brown’s rediscovered ally, sits on 35 of the 43 cabinet committees. When it comes to truly momentous decisions, such as going to war, traditionalists believe the full cabinet must be the forum for informed discussion. In relation to Iraq – as Lord Butler told this newspaper (p13, CSW 3 June) – that didn’t happen.

Would the Tories do anything differently? Their rhetoric in opposition would suggest so: most leading figures in the party have taken the opportunity to criticise Labour’s approach to cabinet government. Shadow Cabinet Office minister Francis Maude told us last year (p1, CSW 7 October) that Labour has centralised far too much power , and David Cameron has repeatedly promised to restore the strength of the cabinet as a discussion forum. The leadership also embraced a report by Ken Clarke which complained that under Labour, “Cabinet government has been all but destroyed: most ministers have become little more than the presentational vehicles of political appointees in Number 10”.

But these declarations sit uneasily with recent reports that the party is considering turning Number 10 and the Cabinet Office into a centralised hub directing Whitehall, populated by cabinet ministers in an operation reminiscent of the White House. Peter Riddell dismisses these plans as “unworkable and undesirable”, and George Jones of the LSE says such a move would centralise power in a way not seen since the early 19th century.

The Lords investigation into the centre of government continues, and should produce some interesting conclusions – not just on the operation of the cabinet itself, but on relations between the ‘holy trinity’ of central departments: the Treasury, the Cabinet Office and Number 10. As for what the current cabinet secretary thinks about it all, we won’t have to wait long – he’s giving evidence on November 4.