British officials have raised the threat level regarding a possible attack on the mainland by Irish Republican dissidents.

The government Wednesday raised the threat level from "moderate" to "substantial" meaning that a terrorist attack is viewed as a "strong possibility."

Home Secretary Theresa May said security officials raised the level because of the "continuing threat from dissident Republican activity."

Several dissident Republican groups in Northern Ireland remain opposed to the peace process supported by Sinn Fein, the major Catholic-backed party in Northern Ireland.

The overall threat level to Britain remains at "severe" mostly because of the danger posed by Islamic State extremists and their allies in Britain.

May called for the public to be vigilant at a time of deepening threats.