The West can only contain not defeat militant groups such as al-Qaeda, the head of the UK's armed forces has said.
General Sir David Richards, a former Nato commander in Afghanistan, said Islamist militancy would pose a threat to the UK for at least 30 years.
But he told the Sunday Telegraph newspaper a clear-cut victory over militants was not achievable.
The BBC's Frank Gardner said the comments reflect a "new realism" in UK and US counter-terrorism circles.
Our security correspondent said such an admission five years ago might have been considered outrageous and defeatist.
Gen Richards, 58, took over as chief of the defence staff last month, after a spell as head of the British army.
'Secure lives'
He is due to lay a wreath at the Cenotaph in London later as part of the UK's Remembrance Sunday commemorations.
In his Sunday Telegraph interview, Gen Richards expressed confidence that al-Qaeda could be contained to such an extent that Britons could lead secure lives.
Gen Richards said: "In conventional war, defeat and victory is very clear cut and is symbolised by troops marching into another nation's capital.
"First of all you have to ask: do we need to defeat [Islamist militancy] in the sense of a clear-cut victory?
"I would argue that it is unnecessary and would never be achieved."
Gen Richards added: "But can we contain it to the point that our lives and our children's lives are led securely? I think we can."
He said the best weapon in the battle against al-Qaeda was the use of "upstream prevention" and the promotion of "education and democracy".
He drew similarities between militant Islam's "pernicious ideology" and that of Nazi Germany.
Gen Richards also admitted the Afghan people were beginning to "tire" of Nato's inability to follow through with its promises to the country.
Britain has lost 343 soldiers in Afghanistan since 2001 but Gen Richards said their sacrifice had been worth it.
Security lapse
He said he saw no reason for Britain to intervene militarily in other countries like it had in Iraq and Afghanistan but added: "It would be barmy to say that one day we wouldn't be back in that position."
Gen Richards comments came as the Foreign Office apologised to a group of MPs after a fact-finding trip to Afghanistan and Pakistan had to be called off because of a security lapse.
Next week's visit by members of the Commons defence select committee was cancelled after an unencrypted e-mail was sent out by an embassy official in Kabul, prompting fears that the MPs' safety may have been compromised.
The Foreign Office said it would be trying to rearrange a visit for the MPs.
A Foreign Office spokesman said: "We have offered our apologies for this regrettable lapse in our procedures and have assured the committee that we will do all we can to arrange a successful visit in the future."
