Jonathan Evans, the director general of British intelligence services MI5, warned a summit of intelligence professionals last week that his agency has seen "a persistent rise" in terrorist activity associated with Northern Ireland dissidents during the past three years.
"The Security Service has raised the threat level from 'moderate' to 'substantial'," the Friday warning read. "This means an Irish-related terrorist attack is a strong possibility."
London's Daily Mail reports Friday that authorities in the country believe a splinter group of the Irish Republican Army has the means to acquire explosives and boasts of terrorist organizers in its ranks.
"This is the first time an Irish-related threat assessment has been published," the warning added.
The so-called Real IRA recently said it would target financial institutions in London for "financing Britain's colonial and capitalist system," the Daily Mail adds.
British Home Secretary Theresa May said the rise in the terror alert meant that there was a strong possibility of an attack.
"The balance we aim to strike is keeping people alert but not alarmed," she said in a statement. "I would urge the public to report any suspicious activity to the police and security services in their continuing efforts to discover, track and disrupt terrorist activity."