* EU defence ministers agree 17 projects to develop together
* Britain seeking "third party" involvement due to Brexit
BRUSSELS, March 6 (Reuters) - A group of European Union countries agreed on Tuesday to develop their first joint defence projects under a pact that excludes Britain, giving London a taste of life outside the bloc's foreign policy decision-making process.
The 25 signatories to the pact also delayed a decision on whether to let non-member states join the projects, prolonging uncertainty over any future role for Britain after it leaves the EU next year.
As Europe's biggest military power along with France, Britain is central to European security efforts but has long blocked defence integration, fearing the creation of an EU army.
Nevertheless, Britain is concerned about being left outside the new cooperation pact and missing out on weapons projects. Prime Minister Theresa May wants to reach a "security treaty" with the EU by 2019.
Defence ministers from the pact's signatory states, which comprise all but three of the current EU members, signed off in Brussels on 17 collaborative projects. These include a European armoured infantry fighting vehicle, underwater anti-mine sensors and a European medical command.
The eventual aim of the Permanent Structured Cooperation pact is to develop and deploy forces together, backed by a multi-billion-euro fund for defence research and development that is now under negotiation.
Denmark, which has opted out of most EU military matters, and neutral Malta are the others not taking part.
Its strongest backers - France, Germany, Italy and Spain - hope that by achieving a long-held ambition to develop national defences together, the EU will save money by putting an end to competing national industries.
Spanish Defence Minister Maria Dolores de Cospedal played down some U.S. concerns that Europe might overlap with what the transatlantic alliance is doing. "There is no suggestion of duplication with NATO," she said.
But in some areas, the Europeans do seek "strategic autonomy" from the United States to be able to face threats on Europe's borders in North Africa and the Sahel.
At their meeting, the ministers delayed until the end of this year any decision on whether to allow non-EU countries to join future defence projects. That was despite expectations of a decision by the end of June.
One senior EU official said ministers want to see more progress in Britain's exit negotiations with Brussels given the sensitive nature of defence cooperation with London.
In an EU document approved by ministers, any outside involvement in the pact will be "of an exceptional nature" and only if a country meets certain conditions, such as bringing sufficient financing and technical know-how. (Reporting by Robin Emmott; editing by David Stamp)