There have been 27 paramilitary-style attacks in Northern Ireland so far this year amid heightened concerns about the risk posed by dissident republicanism, Northern Ireland Secretary Theresa Villiers has said.
The threat level from Northern Ireland-related terrorism was raised from moderate to substantial in May, meaning a strike against the mainland is "a strong possibility".
Ms Villiers said dissidents have been found with guns and bomb-making equipment and are known to want to target police officers, soldiers and prison officers.
In an urgent written statement to the British Parliament, she said: "So far this year, the PSNI have recovered terrorist items including firearms, ammunition and bomb making equipment.
"There have been 59 arrests, of which eight individuals have been charged for terrorist related offences.
"The lethal nature of the threat posed by terrorist groupings was demonstrated in March when prison officer Adrian Ismay died as a consequence of injuries he sustained when an improvised explosive device functioned under his vehicle as he left his Belfast home for work.
"Adrian Ismay's death is first and foremost a tragedy for his family and friends. But it also serves as a stark reminder of the ongoing risks faced by prison officers, police officers and members of the armed forces, some of whom have been very fortunate to escape injury in other terrorist attacks, both on and off duty."
She said republican and loyalist organisations conduct "brutal" criminal attacks in a bid to exert control over their communities.
Between January and May of this year, there were 27 paramilitary-style attacks, three of which were fatal.
She said tackling terrorism "is the highest priority for this Government" but warned that "further potentially lethal attacks are highly likely".