MI5 knows of 30 terror plots threatening the UK and is keeping 1,600 individuals under surveillance, the security service's head has said.
Eliza Manningham-Buller rarely speaks in public
Dame Eliza Manningham-Buller warned the threat was "serious" and "growing".
She said future attacks could be chemical or nuclear and that many of the plots were linked to al-Qaeda.
Prime Minister Tony Blair said the terrorist threat was "very real" and spoke of "poisonous propaganda" warping the minds of young people.
MI5 has increased in size by nearly 50% since 9/11 and now stands at roughly 2,800 staff.
But according to Dame Eliza the current terror threat will "last a generation" and her concern is that even with MI5's rapid growth, the security service will not be able to investigate nearly enough of activities it deems to be suspicious.
She said hard choices would have to be made about resources.
"I wish life were like Spooks [the TV series] where everything is, a, knowable, and, b, solvable by six people," she explained.
Dame Eliza's warning comes days after a UK man was sentenced to at least 40 years in jail for planning a series of attacks.
Attacks planned by Dhiren Barot, 34, from London, included using a so-called "dirty bomb" using radioactive material.
Mr Blair also said he agreed with Dame Eliza's comments that the terrorist threat would last for a generation.
"I've been saying, as you know, for several years that this terrorist threat is very real, it's been building up over a long period of time.
"It's not just in this country, as we've seen recently from incidents in India, France, other parts of the world. This is a threat that has grown up over a generation."
In response to Dame Eliza's warning, Massoud Shadjareh, of the Islamic Human Rights Commission, said he accepted there was a terrorist threat but it had to be put into perspective.
"Over 1,000 arrests have been made under anti-terrorism since 9/11 and out of those, 27 have been found guilty. Out of those 27, only nine have been Muslims," he said.
Dame Eliza, who rarely speaks in public, gave a speech to a small audience on Thursday, detailing what she believes her organisation and the UK is facing.
She said that, since the 7 July bombings, five further major conspiracies in the UK had been thwarted.
"Today, my officers and the police are working to contend with some 200 groupings or networks, totalling over 1,600 identified individuals - and there will be many we don't know - who are actively engaged in plotting, or facilitating, terrorist acts here and overseas," she said.
"Today we see the use of home-made improvised explosive devices.
"Tomorrow's threat may - I suggest will - include the use of chemicals, bacteriological agents, radioactive materials and even nuclear technology."
'British foot soldiers'
Out of the 200 or so groups being watched by MI5, a smaller subset are of the highest priority because it is feared that they are plotting actual attacks.
"We are aware of numerous plots to kill people and to damage our economy. What do I mean by numerous? Five? 10?
"No, nearer 30 that we currently know of.
"These plots often have linked back to al-Qaeda in Pakistan and through those links al-Qaeda gives guidance and training to its largely British foot soldiers here on an extensive and growing scale."
She added that of the 30 plots some may turn out to be less credible or advanced but it would be hard to be sure until they are fully investigated.
Tory security spokesman Patrick Mercer echoed her call for more resources, saying "we just don't have enough spooks and secret agents to make sure that our country is as safe as it could be."