Scotland Yard are hunting three men who are believed to have absconded from control orders imposed under the Prevention of Terrorism Act.
Lamine and Ibrahim Adam failed to contact a monitoring company
Police say Lamine Adam, 26, his brother, Ibrahim, 20, and Cerie Bullivant, 24, have been missing for more than 24 hours and may be together.
Members of the public are being warned not to approach them, but to call 999.
The Adams' brother, Anthony Garcia, was jailed for life last month for his part in the fertiliser bomb plot.
Home Secretary John Reid is due to make a written statement on Thursday, but the Tories say the news is "shocking."
Control orders place a type of house arrest on those suspected of international terrorism.
Police launched the appeal for help tracing the men by releasing their names, descriptions and pictures after all three failed to make contact with authorities.
Cerie Bullivant may have cut his hair off
The Adam brothers did not contact a monitoring company on Monday evening.
And Mr Bullivant is understood to have been missing since Tuesday morning when he failed to report to his local police station.
Head of the Metropolitan Police's Counter Terrorism Command Peter Clarke said inquiries were being made to trace the three men.
"It is possible that the public can help us to trace them. They have breached the conditions of their control orders, which is a serious crime," he said.
The Home Office said there are 17 people currently on control orders. In total, six people have absconded from control orders.
Earlier on Wednesday, in a break with the practice adopted after the previous incidents, Mr Reid asked the High Court to lift the three men's anonymity to help with the police investigation.
Police described Lamine Adam as of north African origin, 6ft, slim, with a light complexion, short dark brown hair and last seen with a short beard.
Ibrahim Adam is also of north African origin, 6ft 2in, slim with a light complexion and short dark brown hair.
It later emerged their brother, Anthony Garcia, 25, was among those jailed last month at the Old Bailey for his part in a plot to build a homemade bomb capable of killing hundreds, the Met Police confirmed.
They said Cerie Bullivant was white, 5ft 7in, slim with receding brown hair and brown eyes. It is understood he recently cut his hair very short.
Shadow home secretary David Davis, said: "It is shocking that yet another three have absconded.
"John Reid's primary responsibility is the protection of the public. This consistent yet gravely dangerous failure to carry out that duty continues to threaten the safety of the public."
The Chairman of the Commons home affairs select committee, the Labour MP John Denham, told the BBC that control orders had always been an imperfect solution.
"They were brought in because the courts prevented the government from jailing people who were believed to be terrorists, sometimes had a terrorist record overseas; we were stopped from jailing them because we didn't have the evidence to convict them here."
Liberal Democrat home affairs spokesman Nick Clegg called for a review of the "increasingly discredited" system of control orders.
Shami Chakrabarti, director of human rights group Liberty, said the control order regime compromised security and fairness.
"Innocent people are punished without trial and the potentially less innocent easily escape," she said.
People with any information can call the confidential anti-terrorist hotline on 0800 789 321.