IT is difficult to conceive of security measures which may have prevented yesterday's atrocity in Madrid.

Systems of public transport offer an easy target for determined assassins. A co-ordinated attack on crowded commuter trains is bound to lead to huge casualties.

This morning we share in the grief and the sorrow felt by the people of Spain. Not simply because it is a country many of us visit and know so well and is a fellow member of the European Union. But also because we know how vulnerable our major cities are to such an act of terror.

Whether it was the work of Eta or another organisation, yesterday's outrage demonstrates to us all the very real dangers posed by terrorists.

The cowardly act of leaving explosive devices on crowded trains can do nothing to promote the cause pursued by the terrorists. Nor can it make any government or public opinion more receptive to their grievances.

With such unfathomable motives, dealing with terrorism is so intractable.

Yet, the experience in Northern Ireland shows that all hope is not lost.

Public opinion must remain solidly opposed to terrorism, whatever the level of carnage, intimidation and fear.

And there must be no diminution in the surveillance of terrorist activity, and bringing to justice the perpetrators of terrorist crimes. We must make the lives of determined and ruthless terrorists as difficult as possible.

Only by refusing to give in to terrorism, can there be any hope that the terrorists will eventually see the futility of their murderous ways and seek a peaceful and political settlement.

Atrocities like the one in Madrid test our resolve, but our resolve must remain stronger than ever.