By Dominic Casciani
We may have only seen the first round of the struggle, says Prof Clarke.
The UK may face a new generation of terrorists more dangerous than the semi-trained "amateurs" now in jail, a security think tank's chief has warned.
Michael Clarke, of the Royal United Services Institute, said recent years may come to seem like a "golden age" of successes against violent extremism.
Home-grown jihadists would not wither away, he warned in a think tank paper.
He argued that convicts in UK jails could turn them into "universities of terror", like in other countries.
Around 200 people have been jailed for terrorism-related offences in the UK since the 9/11 attacks on the US in 2001.
Professor Clarke said the police and MI5 could be rightly proud of how they had so far confronted the threat inspired by al-Qaeda.
He said many of the UK jihadists now in jail had shown themselves to be "resolutely amateur".
"Intercept evidence shows that jihadists in the UK talk like terrorists operating in professional cell structures but do not normally act that way," said Prof Clarke.
"Most of the plotters have left a trail of forensics behind that has led some police professionals to predict that this will seem like a golden age of counter-terrorism - when we were both successful and lucky.
"The tradecraft of UK jihadi terrorists is extremely variable. For the movement as a whole this is not a problem.
"Amateurs are as dangerous as professionals if they are lucky, and if there are enough amateurs plotting, some of them will be lucky. Those who are not keep the security services stretched and public anxieties high."
Prof Clarke said Nato's operations in Afghanistan had limited al-Qaeda's ability to direct bomb plots in the UK - but he warned recent years had been only "the first round of the struggle".
The dangers could now increasingly come from home-grown extremists, particularly alienated young Muslims susceptible to grooming.
"An evolution in the recruitment and techniques of terrorist cells in the UK is entirely possible. The convictions, of which the security services can be proud, will have their own longer-term consequences for which the government must be prepared.
"The phenomenon will not wither away in the near future: it is likely to be generational."