"Another great plot" is being investigated by the authorities, counter-terrorism minister Lord West warned today.
"There is another great (terrorist) plot building up again and we are monitoring this," he told peers during a debate on new security laws.
Lord West, the former head of the Royal Navy, revealed the existence of the plot as the House of Lords continued debating the Counter Terrorism Bill.
He gave no more details of the threat.
His warning came less than 24 hours after peers forced the Government to abandon plans to extend maximum pre-charge detention for terror suspects to 42 days.
Home Secretary Jacqui Smith dropped the measure from the Bill after it was defeated in the upper house by 191 votes. But she warned critics that they were exposing the country to a greater risk.
Lord West was speaking during exchanges after repeating the Home Secretary's emergency statement last night setting out her plans to keep the 42-day proposals in a separate piece of legislation to be introduced if needed in face of an emergency.
He told peers: "Some of the measures that we have put into place in the past 15 months have made us safer, but that does not mean that we are safe.
"The threat is huge. It dipped slightly and is now rising again within the context of 'severe'. There are large complex plots. We unravelled one, which caused damage to al-Qa'ida and the plots faded slightly.
"However, another great plot is building up again, which we are monitoring.
"We have done a great deal to protect ourselves and to look after our water supplies, our resilience, underground trains, our preparedness and communications.
"We have done all the things that we need to do, but the threat is building - the complex plots are building."
The current threat level in the UK is assessed officially as "severe" - the second highest rating.
In a speech last year, MI5 director general Jonathan Evans said 2,000 individuals had been identified in the UK who were considered to pose a direct threat to national security because of their support for terrorism.
And he warned that there could be as many again which MI5 did not know about.
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