CONTINUATION: 'His great mistake was to identify the right problem

through the late 1990s and into the early 2000s.

In the aftermath of the September 11 attacks, Mr Blair's allies let it be known that he had spent that summer reading the Koran and was seeking to understand what he saw as a "perversion" of Islamic teaching.

Professor Paul Wilkinson, of the Centre for the Study of Terrorism and Political Violence, says that it is "credible" that Mr Blair understood the problem and says that he was effective in his initial response to the New York attacks.

But just as al-Qa'ida was on the "back foot" after the enforced removal of the Taliban regime in Afghanistan, Professor Wilkinson says the invasion of Iraq gifted it a lifeline. "Nobody could credibly argue that [the invasion of ] Iraq caused terrorism, but to pretend that it does not exacerbate it was really foolish."

Crispin Black, a former intelligence officer, says that Mr Blair was warned repeatedly about the consequences for domestic terrorism of the Iraq adventure by the intelligence services and the Foreign Office. "Regardless of what you think about the rights and wrongs of the Iraq war, the question is, when warned, what did Blair do to secure the home front?"

The answer to that question, Mr Black says, is not one that shows the outgoing Prime Minister in a flattering light.

He says that Mr Blair was "badly served" by his security and intelligence services both in the run-up to the Iraq war and over the 7 July bombings, but his "corrupting" influence would cause lasting damage. The dodgy dossier had broken a "covenant of trust", undermining the credibility of all subsequent intelligence warnings. "Time and again the warnings have been shown to be exaggerated or wrong. The tragedy is we won't believe them when they are right."

The exact state of the current threat to Britain is, by its nature, unknowable but Professor Wilkinson is gloomy: "The trends and the emerging trends confirm that this is going to be a difficult problem for a very long time yet."

Mr Blair's great mistake, suggest the experts, was to identify the right problem but then fail to apply the correct solution.

Mr Black says that the Prime Minister was right to identify the Israel-Palestine conflict as an "open wound" in which extremism was festering. But in reaching for a military solution to Iraq - which in any case was not a part of the al-Qa'ida equation in 2003 - he handed terrorists a new cause and a training ground. The extent to which British terrorists are being trained under the cover of the Iraq insurgency is unclear, but there is no doubting the terror traffic between the UK and Pakistan. In a recent talk to the Policy Exchange think tank, Dame Pauline Neville-Jones, formerly chairman of the Joint Intelligence Committee (JIC), said that the UK was nowhere near disrupting this "transmission belt" between Lahore and London.

Dame Pauline is scathing about Mr Blair's efforts to win the "hearts and minds" battle that will be needed to turn new generations away from terror. Counter-terrorism , she says, is better framed in terms of a criminal conspiracy than a "war on terror", which lends participants the dignity of being "soldiers" for their cause. And seeking to engage a "Muslim community" through representative bodies is an approach as outdated as the colonialism of which it smacks.

One enduring legacy of the Blair era will be the massive increase in surveillance and diminution of civil liberties. The unhappy saga of control orders - a device only introduced because detention of terror suspects without trial was ruled unlawful and which was itself then rejected by the law lords - showed how cheaply Mr Blair's government has handed propaganda victories to Britain's enemies. He may not have been responsible for Abu Ghraib or Guantanamo but has pushed the law to its very limit, says the former JIC chair.

"The price is internal surveillance to an unprecedented degree ... there has not been this degree of penetration of our society by forces hostile to the state since Sir Francis Walsingham was pursuing Catholic plotters in the 16th century," Dame Pauline says. "We are about at the acceptable limit of restraints on freedom of speech and association, such as the restrictions on demos near Parliament, and the curtailment of habeas corpus."

A little over a year into his premiership,dissident republicans detonated a car bomb in the middle of the Northern Ireland border town of Omagh with scant warnings. They killed 28 people, including nine children. Tuesday will be a celebration that threat has cleared. The darkness that followed hard behind will hang heavily over Britain long after Mr Blair has gone.

A decade of attacks and near misses

AUGUST 1998 Dissident republicans detonated a car-bomb (right) on a busy Saturday in Omagh, Northern Ireland, killing 28 people, including nine children.

11 SEPTEMBER 2001 Nineteen terrorists hijacked four commercial aircraft, crashing into the World Trade Center towers and the Pentagon. Nearly 3,000 people died. The event prompted many countries to introduce antiterrorism legislation.