Britain has been just days from a major terror attack, with five plots foiled in the last four months, Met Police chief Sir Bernard Hogan-Howe.
Terrorists have been 'very close to hurting somebody badly, or killing them' since the summer and there is 'no doubt' that there are extremists in the UK as dangerous as the Muslim gunman behind the Sydney café siege, the Commissioner warned.
And he claimed officers are struggling to cope with the mountain of digital files seized in police raids, with recent cases seeing terror suspects hours from being released before crucial evidence was found for them to be charged.
David Cameron warned Britain could be hit 'at any moment' by a Sydney-style terror attack.
Two hostages died and Islamic extremist Man Haron Monis was killed after a 16-hour siege in the busy financial district of the Australian capital.
Cameron said it was only through the efforts of the security services that plots to murder UK police officers 'in cold blood' had been foiled in recent months.
But Sir Bernard insisted that the police need extra resources to deal with the growing terror threat in Britain. In a wide-ranging interview, the country's most senior police chief:
- Revealed five plots have been thwarted in the past four months, including some which were just days away from killing and maiming on the streets of Britain
- Demanded an extra £50million to fund counter-terror operations, including tracking suspects and analysing digital evidence
- Warned that children as young as 15, including growing numbers of girls, are travelling to fight in Syria, using Spain as a stop-off to Turkey to avoid detection
- Appealed to members of the public, and even other criminals, to report any unusual behaviour which could be linked to terrorism
Sir Bernard told LBC Radio: 'In terms of plots over these last four months really it's probably five. We have arrested 35 people.'
The dramatic warning comes amid growing fears of a so-called `lone wolf attack` in the UK.
In Sydney, the Iranian asylum seeker Monis struck terror into the heart of the city after taking 17 staff and customers hostages inside the Lindt cafe in Martin Place.
The 50-year-old Muslim gunman is believed to have fired the first shots, which sparked teams of heavily armed police to swoop on the café at 2am on Tuesday.
Asked if there were people in Britain as dangerous as Monis, Sir Bernard told LBC radio: 'There will be, there's no doubt. You look at what happened to Lee Rigby.
'There are people out there who are dangerous, and that's why we keep arresting them because we need to stop them hurting people.
'And the operations we have talked about earlier, they have been very close to hurting somebody badly, or killing them. So they are a threat. Very, very close.'
Pressed on how close the plots had been to being carried out before being intercepted by police, he replied: 'I'm acknowledging days.'
Terror suspects can be held for up to 14 days before they must be charged or released.
Sir Bernard revealed: 'The last couple of case we have got right to the wire, to 13 days, I fact 13 and a half days, another one 12 days, and then we've found the thing we needed to prove the charge.
'Had we not got that we'd have had to release on bail. So for those reasons we need some help.'
The government last month announced that an extra £130million would be made available over the next two years for the security services to 'monitor and disrupt those self-starting terrorists'.
But Sir Bernard said Scotland Yard needed more money to cope with the rising threat level and the increasingly complex use of technology by criminals and terrorists.
Police and security officials have previously warned it is 'almost inevitable' that Britain will be hit by an attack by fanatics who have been 'militarised' by Islamic State.
Sir Bernard warned that have a 'radicalised individual' with weapon taking many people hostage is 'a terrifying prospect' which is 'very difficult to guard against'.
He stressed that the best defence that the police has is good intelligence.
'It means members of the public tell us when they are worried about someone. Other criminals tell us when they are worried about people.
'We get some of our best information from criminals, for various motives, frankly sometimes financial, but sometimes they don't like them, sometimes they're frightened of them.
'We get the best information from people in that area.'
Last night the Prime Minister warned it was much harder for the authorities to prevent attacks by fanatics who are 'self-radicalised on the internet' than to tackle known extremists who had travelled to the Middle East.
Cameron, appearing before senior MPs at the Commons Liaison Committee, said: 'The threat we face definitely includes those sort of self-starting, sometimes quite random attacks that could happen at any moment in Britain.
'We've seen over the last few months there have been a series of plots that have been detected and prevented that would have seen police officers or other authorities murdered in cold blood.
'It's thanks to the brilliance of our security services that these things have been prevented.'
But he added: 'We can't count on them to prevent it every time because it is one thing understanding the terror networks coming out of Pakistan or Afghanistan or Iraq and Syria, trying to monitor what they're doing, who's going and who's returning. That's one thing.
'But people who are self-radicalised on the internet, who then suddenly do appalling things, that is much more difficult to prevent.'.
Scotland Yard has requested an extra £50million to track and prosecute terror suspects.
Sir Bernard Hogan-Howe revealed he has asked the government for 'more help', including staff to trawl millions of digital files seized in counter-terror raids.
He likened trying to find conclusive evidence of extremism to looking for a needle in a haystack, adding: 'The last job we had 10 haystacks and weren't sure which one the needle was in.'
Sir Bernard said: 'Have we got enough resources? Well we have asked the government for more help.
'We are losing money, but in this area we have got to invest so we have asked for around £50milion for across the country. We are waiting for an answer on that. We need it.'
He said counter-terror operations 'use lots of people' and cost a lot of money.They are resource intensive when trying to track suspects around the clock.
But once someone has been arrested, officers facing a daunting task in analysing their computer files for evidence.
Sir Bernard added: 'When we arrest them what we have found is we are seizing huge amounts of digital evidence, computers, laptops, all the things we all have.
'One of the operations, the last one I think, one person had about 19 devices. Now you think that's ridiculous. But you think how many devices you have at home.
'We get all that, then we've got to download it, and then having downloaded it we have got to find the stuff.
'Sometimes you are looking for a needle in a haystack. The last job we had 10 haystacks and weren't sure which one the needle was in. It's really hard. So we are getting backlogs in the digital retrieval.'
The government insists that it does not break down the counter-terrorism budget into individual lump sums it is only ever published as a single total figure.
Officials also refused to discuss the details of funding requests made by 'operational partners'.
Mr Cameron last month announced that an extra £130million would be made available over the next two years for the security services to 'monitor and disrupt those self-starting terrorists'.
The PM's official spokesman said: 'We have already protected funding for counter terrorism policing due to the ongoing threat posed to the UK by terrorism and more recently the Prime Minister announced an additional £130million to strengthen counter-terrorism capabilities.
'This will include new funding to enhance our ability to monitor and disrupt terrorists and some of this funding will go to counter-terrorism policing.'
A Home Office spokesman added: 'Public protection is the first responsibility of any government and we are committed to ensuring the police and other agencies have the resources they need to respond to changing threats.
'While all public services must constrain their spending, we have protected funding for counter terrorism policing due to the ongoing threat posed to the UK by terrorism.
'The Prime Minister has also announced an additional £130million which will be made available over the next two years to strengthen counter-terrorism capabilities. This will include new funding to enhance our ability to monitor and disrupt terrorists and additional resources for programmes to prevent radicalisation.