Tony Blair has arrived in Brussels from his historic talks with Libya's Colonel Gaddafi for a summit of EU leaders.
EU constitution may be back on track
Security is set to dominate the meeting, which comes in the same week as a memorial service for the victims of the Madrid train bombings.
European leaders have agreed a range of security measures, including the naming of an anti-terrorism co-ordinator.
They have also set themselves a mid-June deadline to broker a deal on a landmark EU constitution.
Talks about such a constitution fell apart in December with disagreements over voting rights.
But with 10 new states set to join the EU on 1 May there is new momentum towards an agreement.
The first session was on security with plans for greater mutual assistance in the event of a terrorist attack.
Dutch politician Gijs de Vries, who starts work on Monday as the new anti-terrorism co-ordinator, will head the renewed effort pooling the gathering of Europe-wide intelligence.
A spokesman for the UK Government said: "From everything we have seen, he seems a very good choice.
"The most important thing is not who he is but that he does the job properly."
New measures agreed include monitoring data from mobile phone calls and a single EU-wide arrest warrant.
The leaders of the 25 current and future members agreed a number of steps aimed at showing Europe united in the face of terrorism:
- implementing already agreed counter-terrorism measures, such as a pan-European arrest warrant, harmonising penalties for terrorist crimes and freezing of assets held by outlawed groups
- retaining all telecommunications data, including mobile phone records, for an agreed minimum time to enable intelligence agencies to track calls
- stepping up security at EU ports.
But the BBC's William Horsley in Brussels says persuading EU governments to share more of their sensitive information will not be easy.
There are also significant differences over how to respond to the terror attacks blamed on Islamic militants, as well as how to treat Europe's growing Muslim population.
Earlier European Parliament President Pat Cox appealed to heads of government before they arrived in Brussels to find the political will to complete negotiations on the constitution before June's EU-wide elections.
He said: "The time for a constitutional treaty settlement has come, and that time is now.
Irish Prime Minister Bertie Ahern, the current EU president, signalled that "we have a changed atmosphere" and that "we all want to see a new constitution in place as soon as we can".
Last year's negotiations collapsed after accession state Poland and existing member Spain united to block a deal on voting rights.
Meanwhile France and Germany want a new system to reflect their bigger populations.
Poland has hinted it is prepared to compromise and a change of leadership in Spain could mean the proposed constitution regains momentum.
If that happens, Tony Blair will come under pressure from the Lib Dems and Tories to hold a referendum on the issue.