The risk from terrorists in the Christmas period is very high indeed and the struggle against Islamic terrorism will last at least 30 years
Mr Reid echoed the view of MI5 that there are around 30 major terrorist plots under way and the terrorists only "have to be lucky once".
He told the GMTV Sunday Programme that an attempted attack over the Christmas period was "highly likely", adding: "We know that the number of conspiracies of a major type are in the tens — 30 or round about that.
"We can never guarantee that we will get 100 per cent success but we do get 100 per cent effort from the security services."
Mr Reid said the current UK terror assessment posted on the Government's intelligence website was "severe", the second highest level, one short of the level at which an attack is imminent.
The Home Secretary said he did not think an attack was "inevitable" but added: "The terrorists only have to get through once, as they did on July 7, for us to see the terrible carnage that it causes. Our security services have to be successful on every occasion to prevent that happening.
"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."
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