Analysts believe the service will be forced eventually to downgrade its activities in Northern Ireland and will seek a new role, probably countering the drugs trade and organised crime.
But a security source yesterday predicted that MI5 would continue to have a significant role in monitoring Irish terrorist splinter groups even if there were long- term peace.
MI5 devotes 44 per cent of its resources - about 2,000 officers with an estimated annual budget of pounds 150m - to countering Irish and domestic terrorism.
A security source said yesterday: 'There's been a major step and everybody hopes that it will bring permanent peace. But there are a number of maverick groups, such as the INLA (Irish National Liberation Army), who may carry out attacks. It's important for the Government to know who they are and it will be important to monitor their activities. It's premature to talk of winding down.'
The source added: 'If there's a resumption of violence in 10 years' time, does intelligence start from scratch or is it better to keep a watching brief?'
A significant drop in the security service's operations in Ireland will not take place for five to ten years, according to Professor Paul Wilkinson, head of international relations at the University of St Andrews, Fife.
He said: 'Managing the peace process can lead to intensified violence as people and organisations become desperate or fear a sell-out. There is going to be a very real danger for quite some time - MI5 is not going to suddenly disappear even if the ceasefire holds.'
On the future role of MI5, he said: 'It's unrealistic to think they will stand aside from organised crime and drugs, as they are such huge problems.'Reuse content