The change was announced just weeks after a failed plane bombing in the United States, and days ahead of two major international conferences on Yemen and Afghanistan in London.
"The Joint Terrorism Analysis Centre has today raised the threat to the UK from international terrorism from substantial to severe," Johnson said, referring to the assessment unit within the MI5 domestic intelligence agency.
"This means that a terrorist attack is highly likely, but I should stress that there is no intelligence to suggest than an attack is imminent."
In a television statement shortly afterwards, Johnson refused to say whether the amended threat assessment -- to four in a five-level scale -- was linked to the failed Detroit plane bombing on December 25.
"We never say what the intelligence is," he said, adding: "It shouldn't be thought to be linked to Detroit or anywhere else for that matter."
But a US official, who requested anonymity, said "that's the implication".
The US Department of Homeland Security said the announcement brought Britain in line with US security measures introduced "over the last few weeks".
Johnson said the JTAC "looks at all factors and no one should draw any assumptions from this", adding that the higher threat level meant Britain put "more resources in, we heighten the state of vigilance".
In a statement issued by his office, he added that the threat level, which has been made public on MI5's website since August 2006, was kept "under constant review".
The analysis centre "makes its judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK and overseas", he said.
Johnson stressed that Britain continues to face a "real and serious threat" from international terrorism and urged the public to remain vigilant.
The threat level was last at "severe" on July 20, 2009, when it was downgraded to substantial, suggesting an attack remains a "strong possibility".
Prime Minister Gordon Brown on Wednesday reiterated the threats Britain faced as he unveiled new security measures sparked by the attempt to blow up a US airliner flying into Detroit, which has been claimed by Al-Qaeda.
"We know that a number of terrorist cells are actively trying to attack Britain and other countries," he told lawmakers in the House of Commons.
Brown said the "crucible of terrorism" was based on the Afghan-Pakistan border, but noted how the failed Detroit attack also highlighted the threat posed by militants in Yemen.
The alleged bomber, Umar Farouk Abdulmutallab, had reportedly trained in Yemen. He had also studied in London for three years.
Britain has organised a meeting to strengthen international support for Yemen in its efforts against Al-Qaeda, to take place in London on Wednesday, the day before a high-level conference on Afghanistan.
There have been numerous attempted attacks on Britain in recent years, as well as the successful one on July 7, 2005, when four suicide bombers attacked the London transport system killing themselves and 52 others.
Since the threat levels have been made public, they have twice briefly been raised to the top "critical" level, meaning an attack was expected imminently.
The first time was on August 10, 2006, after a series of arrests linked to a plot to down transatlantic airliners, and the second on June 30, 2007, at the time of failed attacks in London and Glasgow.
The two lower threat levels are "moderate", indicating an attack is possible but not likely, and "low", meaning that an attack is considered unlikely.
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