An IRA terrorist attack somewhere in Britain on the day of the royal wedding is "highly likely", British security chiefs have warned.
Strict measures have been put in place around London in preparation for hundreds of thousands of well-wishers on Friday, the Daily Mail reported.
Preparations to counter terrorism have been meticulous in the capital, particularly along the wedding party's procession route.
The Guardian reported that the event will involve one of the biggest security operations in police history, with nearly 5,000 police officers on duty.
However, the focus of a terrorist attack may not be London itself.
The government is braced for attacks in other places after being warned that Irish republican groups could be plotting disruption in Belfast and Londonderry.
The suspected republican attacks would overshadow the wedding between Prince William and Kate Middleton at Westminster Abbey.
A group going by the name of the "Real IRA" has been quoted in several British media, as posing a possible threat.
"Recent intelligence points to a terror attack in Northern Ireland on the day of the wedding. This is a very real threat and described as being highly likely," the Daily Mirror quoted a source as saying.
The threat reportedly emerged in a briefing from British security service MI5 to the Home Office.