Unidentified supporters of jihadist group, the Islamic State (ISIS), have issued warnings via Twitter on Saturday night that Britain could be their next target following the horrendous terrorist attacks in Paris that left 129 people dead and 352 others injured, with 99 in critical condition.

The jihadists added that two other major Western cities — Washington D.C. and Rome — are also in their crosshairs, the Daily Express reported.

The last wide-scale terror attack in London took place in July 2005, when 52 commuters were killed by suicide bombers linked to al-Qaeda on the London transport system.

British authorities said they have put up more stringent security measures to prevent terrorist attacks ever since ISIS militants declared a caliphate last summer.

However, last month Andrew Parker, the head of MI5, admitted the security service can "never be confident" in stopping all terror plots against Britain.

Like Britain, France had also been bracing for a terrorist attack but was still caught flat-footed when the terrorists struck on Friday.

Meanwhile, a terminal at Gatwick Airport was evacuated "for the protection of the public" on Saturday morning after a gun was found. Police confirmed a man from France was being questioned. He was described as "landside" at the airport and "had not checked in or passed through any passport or security checks." He was seen acting suspiciously before throwing a package in a garbage can, reports said.

Concern mounted in Britain as migration expert Tony Smith, a former head of U.K. Border Force, said it was "very worrying" that one of the terrorists responsible for the slaughter of 129 people in the Paris terror attacks reportedly held a Syrian passport, which enabled him to enter Europe from Syria.

"We expected either lone wolves or home grown terrorists radicalised before travelling to Syria and then coming back," Smith said.

"The complexities of this group, that they were fingerprinted, suggests that there have been some breaches of external barriers in Europe."

The finding suggests that at least one of the Paris terrorists might have been posing as a refugee before carrying out his terror mission. He was one of the suicide bombers at the Stade de France and was not known to French authorities. His passport was found near his body, officials said.

Officials have also disclosed that a second man who carried out the killings was likely to have entered Europe in a similar manner.

A Greek government official revealed that the first terrorist, who died in the series of attacks on the French capital, held the documents of a man who had passed into Greece probably by refugee boat last month.

Greece said the holder of the document arrived in mainland Europe from the island of Leros, suggesting he had made his way there by boat.