The Metropolitan Police commissioner Sir Ian Blair has backed the government's new anti-terror law.
Sir Ian Blair believes foreign detainees are a threat to security
Sir Ian warned that if legislation on new control orders is not passed, foreign detainees will pose "a grave threat to public security".
He said that the detainees being held in Belmarsh and other jails could be released as early as Monday.
The government is facing a showdown with the Lords over the law which was approved by the Commons on Wednesday.
Threat to safety
On the detainees, Sir Ian said: "The people in there, that is all of those people, are a threat to the safety of the public of Britain.
"I've have spent the morning re-reading the intelligence in relation to the detainees.
"My sense is all of those currently detained will be subject to control orders."
His comments come just days after former Met chief Sir John Stevens said up to 200 al-Qaeda "terrorists" are operating in UK.
Sir John said any delay in enacting the legislation would bring "comfort" to al-Qaeda and the threat of attacks in the UK is real.
Among the proposals for terror suspects are house arrest, curfew, tagging and bans on internet and telephone use.
The control orders under the new law are intended for cases where it would not be possible to prosecute someone in court - but where intelligence suggested the person posed a terror risk.
The new measures would for the first time allow measures to be taken, without trial, against British citizens suspected of terrorism.