Recent counter terrorism arrests do not mean the threat to the UK has ended, Home Secretary John Reid has said.
Armed police have been out in many UK airports
His warning comes as police continue to hold 23 people over an alleged plot to blow up transatlantic jets.
Mr Reid warned against "complacency or self-congratulation" as the threat posed by extremists is "ongoing", as are efforts to combat it.
Sources have told the BBC potential links between the case and the 7 July 2005 attacks are being investigated.
BBC security correspondent Gordon Corera said officials are also worried other groups, with no connection to the latest alleged plot, could be inspired to carry out their own attack.
As a consequence, it could be several weeks before officials will feel confident enough to lower the threat alert level from critical, our correspondent said.
The Metropolitan Police have warrants for the further detention of 22 of the suspects until next Wednesday.
A hearing regarding the detention of one individual was adjourned until Monday, and one person has been released with no further action.
The suspects were held following raids in London, High Wycombe in Bucks, and Birmingham, early on Thursday.
The person whose warrant hearing was adjourned until Monday will remain in custody over the weekend.
Those held are suspected of involvement in a plot to blow up airliners travelling from the UK to the US, possibly using liquid explosives hidden in hand luggage. Airport security has been stepped up in both countries, with stringent luggage searches and restrictions on the carrying of liquids on board.
British Airways has criticised Heathrow Airport for failing to cope with extra security measures which have caused severe delays and flight cancellations.
Chief executive Willie Walsh said some passengers who checked luggage in were unable to reach the departure gates before their flights took off.
Ryanair has called on the government to provide additional staff to help carry out searches.
Officials in Pakistan, meanwhile, said security forces in the country had also arrested two British men of Pakistani origin in connection with the alleged plot, who were picked up in Lahore and Karachi last week.
The Pakistan Foreign Ministry has identified one of the men as Rashid Rauf.
"There are indications of an Afghanistan-based Al-Qaeda connection," a spokesman said.
NEW THREAT LEVELS
Low - an attack is unlikely
Moderate - an attack is possible but not likely
Substantial - strong possibility of an attack
Severe - an attack is highly likely
Critical - an attack is expected imminently
A Pakistani security official told Reuters he had been put under surveillance after Britain had tipped off Islamabad that he was in Pakistan.
"He has been staying here for quite some time and he has been under strict surveillance since then," the unnamed official added.
Rashid Rauf is thought to be related to Tayib Rauf, 22, who was arrested in Birmingham.
He is among the 19 suspects who have had their assets frozen by the Bank of England.
Britain's security threat level remains at "critical" and investigations are continuing.
Detectives are still searching properties where the arrests took place. Officers have also seized computer equipment from three internet cafes in Berkshire - one in Reading and two others in Slough.
Speaking to chief constables on Saturday, Home Secretary John Reid warned against complacency, saying: "The initial targets - the main suspects - have been successfully apprehended, but all of us know that this investigation hasn't ended.
"I know there is a huge amount to be done and that presents enormous challenges.
"So, we're not yet at the stage where we can or should stop searching. That is why the alert level remains at critical as a precaution."
US President George W Bush justified the increase in security on both sides of the Atlantic in his weekly radio address.
He said air travellers needed to be patient and vigilant.
"We believe that this week's arrests have significantly disrupted the threat, yet we cannot be sure that the threat has been eliminated," he added.
Mr Bush said the alleged plot had the potential to cause "death on a massive scale" as the fifth anniversary of the 11 September attacks approaches.
"We must never make the mistake of thinking the danger of terrorism has passed," he said.