The chances of an attempted terror attack over the Christmas period are "highly likely", John Reid has said.
The home secretary told GMTV's Sunday show the terrorist threat facing the UK was "very high indeed" although he did not think an attack was inevitable.
"We know the number of conspiracies of a major type are in the tens - 30 or round about that," he said.
The UK threat assessment posted on the Home Office and MI5 websites is "severe" - the second highest level.
Mr Reid warned that "the terrorists only have to get through once, as they did on 7 July, for us to see the terrible carnage that it causes.
"Our security services have to be successful on every occasion to prevent that happening."
The battle against terror would continue for "longer than a generation," Mr Reid said.
"When it came to the struggle against republican terrorism in Ireland and in the mainland here, that lasted 30 years.
"There is no indication to me that this is going to be resolved any quicker than that."
Mr Reid also described the difficulties in keeping the public informed of potential terror threats.
"I try to walk the tightrope between being truthful and honest about the threat to the public but, on the other hand, to say we are doing everything possible to combat it and to try to keep our lifestyle as near as possible to the British way of life."
He added that the UK had to be united against terrorism.
"The aim of the terrorists is to divide the community, to pretend that this is a war between Muslims and everyone else, when it isn't.
"In a funny way they reflect the views of the extreme right of politics who argue the same thing - that the big division is between Islam and everyone else.
"It isn't - it's between the terrorists and everyone else and only with that unity can we ultimately defeat them."
Mr Reid said it was important to resolve the Israeli-Palestinian issue because it "fuels international terrorism".
He also praised the role of the security services in protecting Britain against terror attacks.
"We ought to be very grateful to the people in the security services who work night and day to try to protect us.
"We can never guarantee that we will get 100% success but we do get 100% effort from the security services."