The fanatics, in five groups, are now training at secret terror camps in Yemen.
It was there London-educated Umar Abdulmutallab, 23, prepared for his Christmas Day bid to blow up a US jet.
The British extremists in Yemen are in their early 20s and from Bradford, Luton and Leytonstone, East London.
They are due to return to the UK early in 2010 and will then await internet instructions from al-Qaeda on when to strike.
A Scotland Yard source said: "The great fear is Abdulmutallab is the first of many ready to attack planes and kill tens of thousands.
"We know there are four or five radicalised British Muslim cells in the Yemen.
"They are due back within months when they will be under constant surveillance."
The 25 suspects, of Pakistani and Somali descent, were radicalised in UK mosques.
Some had been to university and studied engineering or computer sciences.
Others were former street gang members.
Special Branch monitored them as they flew to Yemen, in the Middle East, from British airports in the spring and summer.
In almost every case, their tickets were paid for in cash and bought less than a week before travel.
The source added: "Imams would have promised them rewards in heaven for becoming suicide bombers prepared to kill Westerners."
PM Gordon Brown and Metropolitan Police Commissioner Sir Paul Stephenson were being briefed.
The warnings came as another Nigerian was last night held in Detroit on the same flight attacked on Christmas Day. It later emerged the man had fallen ill.
Al-Qaeda in Yemen warned the West four days before Friday's attack that a bombing was imminent.
Terrorist Mohammed al-Kalwi issued the video threat in the wake of a Yemeni airstrike on a militant training camp.
Al-Kalwi was reportedly killed in another airstrike on Thursday.
President Barack Obama's administration is to review all airport security.
