"It shouldn't be thought to be linked to Detroit or anywhere else for that matter," Johnson said.

Heightened vigilance

"The appraisal centre keeps the threat level under constant review and makes its judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK and overseas," the statement said.

Terror threat levels

 Critical - an attack is expected imminently
  Severe - an attack is highly likely
 Substantial - an attack is a strong possibility
  Moderate - an attack is possible but not likely
  Low - an attack is unlikely

The increased threat level meant that Britain would put "more resources in, we heighten the state of vigilance", Johnson said.

A Home Office advisory said the threat level meant people should remain on alert for danger, including looking for suspicious bags on public transport.

But it added that they should go ahead with life as normal as the risk of being caught up in an attack was very low.

Crispin Black, a British security analyst, told Al Jazeera that the British government is worried.

"Although the home secretary was vague, he is saying there is more need for vigilance," Black said.

"For the alleged Detroit bomber, there are supposed to be 20 more like him.

"The security alert at India's airports today is very significant, as well as the seven CIA operatives who were killed in Khost in Afghanistan [on December 30]"

"Al-Qaeda and its affiliates are still an on-the-ball organisation. We have to relearn every time that al-Qaeda can come back every time."

The threat level was last at severe on July 20, 2009, when it was downgraded to 'substantial' which means the threat of a attack is a "strong possibility".