At the heart of this exceptionally well researched book is the notion that MI6 has operated as the covert interventionist instrument of British foreign policy. In forensic detail Stephen Dorril shows how, since 1945, our secret service has engaged in what he politely calls "disruptive actions": attempted assassinations (Egypt, Libya), coup d'états (Albania, Iran, Oman), forging Swiss bank account documents (East Germany) and psychological warfare (planting of false information, secret funding of propaganda and smearing opponents).

Many MI6 officers believed in the 50s and 60s that they were the true arbiters of the national interest. As former deputy chief George Young stated: "It is the spy who has been called upon to remedy the situation created by deficiencies of ministers, diplomats, generals and priests." So, for the past 50 years, argues Dorril, MI6 has operated as a state within a state, influencing and manipulating foreign policy to suit its jaundiced view of the world.

Like most western leaders, MI6 believed that nationalism in the Middle East and Africa would inevitably lead to communism. That baseless but popular preconception, alongside a desire to protect US/UK oil interests, coloured their operations, and Dorril catalogues just how far the service was prepared to go to ensure that a government was to its liking. A recurring theme is MI6's dependence on the CIA, whose financial fire-power often gave it the edge, notably in funding European anti-communist networks and technical intelligence-gathering.

But their joint covert actions were not always successful. The 1949 attempt to overthrow the communist regime in Albania ended in abject failure. "It is a gruesome story," wrote defence specialist John Keegan, "made all the more so by the perception, apparently denied to the masterminds of subversion, that the Albanian communists were far more adept at deciding the future of their country than a bunch of romantic meddlers with a public-school education and a free supply of plastic explosives."

A more effective operation was the MI6-controlled coup in Iran, which removed the popular and moderate prime minister, Mohammed Mossadeq, despised by the British because he nationalised the Iranian oil industry. MI6 and the CIA armed, funded and directed the conspirators, and Mossadeq was ousted in 1953.

A crucial component of what MI6 calls "special political action" is the use of psychological warfare. According to Dorril, MI6 planted false stories, secretly subsidised news agencies and radio stations, manipulated opinion polls and smeared opponents by leaking forged documents. Known as "black propaganda", this was a combination of covert news management and sinister dirty tricks.

What emerges from Dorril's exhaustive research is that MI6 has been a law unto itself. A group of senior operatives were obsessed with Britain's decline as a world power, and would resort to any illegal operation to reverse it; they appeared to think foreign policy should be founded on the maxim "God is an Englishman" (as stated by ex-operative Julian Amery). Former MI6 controllers admit that the period 1948-1958 was a horrific dark age, but they claim that the service was cleaned up by Harold Macmillan and has been under ministerial control ever since. Any controversial "black arts" operation needs full Foreign Office sanction. Is this credible?

Dorril provides evidence to the contrary. He cites MI6's involve ment in the 1970 coup in Oman and, to a lesser extent, in Yemen, and relates how, in 1965, MI6 conspired with the CIA to "liquidate" Indonesia's president Sukarno. Margaret Thatcher showed no reluctance in sharing the burden of policing the world; Dorril claims that, in 1980, Thatcher "authorised MI6 to undertake 'disruptive actions' " during the Soviet occupation of Afghanistan.

More recently, former MI6 officer Richard Tomlinson described how, in 1992, he saw an internal document that described a plan to assassinate President Milosevic. Three years later, according to former MI5 officer David Shayler, MI6 plotted to murder Colonel Gaddafi by funding and running Libyan agents who opposed the regime. As more evidence emerges to support Shayler's allegation, it appears that MI6 has not quite relinquished its self-appointed role as an international enforcer of British foreign policy.

If it is to be effective in its new role - investigating the proliferation of nuclear and chemical weapons, tracking drug smugglers and countering money-laundering - MI6 will need to lift the veil of secrecy behind which it has hidden for too long. The lesson of this book is that, unless there is more transparency and accountability, a revival of unofficial bomb-and-blast foreign policy cannot be ruled out.

Dorril, co-author of books on the Profumo scandal and MI5's plot against Harold Wilson, has read an enormous amount. He has interviewed some former MI6 officers who have not spoken before; however, the book relies largely on published sources and there are some gaps. For example, there is not enough on MI6's organisation, its internal structure and how special operations were authorised.

It also suffers from inadequate editing, and Dorril has a tendency to bombard the reader with a bewildering array of facts and names. Given its ambitious scope, though, this is a remarkable achievement and an encyclopedic post-war history which any student of the secret world should read.