The official terrorism threat level in the UK has been raised from “substantial” to “severe”, Home Secretary Priti Patel has announced.
It follows Sunday’s incident in which a passenger in a taxi was killed by a bomb he was carrying.
Last month, a Conservative MP, Sir David Amess, was stabbed to death at his Essex surgery in another incident which has been labelled as a terrorist attack.
The government held a Cobra meeting on Monday, 15 November, and afterwards Ms Patel said: "The Joint Terrorism Analysis Centre are now increasing the United Kingdom's threat level from substantial to severe."
Prime Minister Boris Johnson said of the Liverpool incident: "I can't comment on the details of the case or its motivation but it is a stark reminder of the need for us all to remain utterly vigilant."
Three men have been arrested under the Terrorism Act following Sunday’s incident.
The terrorist threat level was increased in November last year, after an attack in Vienna, and was then reduced again in February 2021.
Philip Ingram, a former British senior intelligence and security officer, said: "It is very difficult in a free society to be able to police the many different ways the terrorist could have accessed the materials needed and instructions to make an explosive device."
He added: "The components can be bought easily, they are all household items and the instructions are widely available in a number of different formats on the web and of course others may have downloaded them."
Mr Ingram said the police were actively tracking more than 800 individuals and had 3,000 on a watch list but he said "tens of thousands" have come to the attention of the security apparatus for various reasons.
He said: "Severe is the next level up in what is called a heightened response - it changes the threat from strongly possible to highly likely and given the amount of recruiting, motivation and training activity terror groups have been carrying out during lockdown and the greater number of gatherings that will be happening in the run-up to Christmas combined with the sophistication of this failed attack, it makes sense to increase the threat level."
According to Mr Ingram, "the most important thing is to raise awareness among the local population and if they see anything suspicious or get a gut feeling that something isn't right then they should report it and the professionals can assess whether there is a real threat or not."
He concluded: "The UK is doing lots right and has foiled six attacks during lockdown - again in a free society it is virtually impossible to eradicate the terror threat completely and the terrorists only have to get it right once whereas the authorities have to be on top of invisible threats 100 percent of the time."