Home Secretary John Reid has estimated the threat from Islamist extremists will last "as long as" the Cold War.
John Reid: Terror threat to last as long as Cold War
That suggests Mr Reid, who said there had been 30 terror plots identified in the UK since 2000, sees the terror threat continuing well into the 2040s.
Mr Reid told MPs he could not "give a guarantee of any timescale.. but my own estimate is this will last probably as long as the Cold War did".
The Cold War began after World War Two in the 1940s and lasted to around 1990.
The same period of time, taking 2001 as the beginning of what was then called the "war on terror", would last to around 2045.
Mr Reid was responding to a question from Conservative MP John Redwood, who asked why Mr Reid thought the terror threat would last a generation - and not longer or shorter.
Mr Reid said it was not an exact science, but given the global scale of the threat and its ideology, he believed the best estimate would be to look at the length of the Cold War.
He said the threat had grown in the UK since 2000, was "serious" and "will, I believe, be with us for a generation".
Answering other questions Mr Reid said there were currently 98 people awaiting trial on terrorism charges in the UK.
He also said the security services had lists of people they believed had attended terrorism training camps.
But, quoting Donald Rumsfeld, he said the problem was the "unknown unknowns".
"Since we know of a considerable number of people in this country who are engaged in conspiracies and we do know that some of those have gone and trained abroad, we must assume that there are others of whom we do not know," he told MPs.