BRUSSELS (Reuters) - The European Union will allow non-members such as Britain and the United States to take part in future joint EU defence projects, but only on an exceptional basis, three EU diplomats said on Wednesday. The decision resolves a long saga over whether Britain, which has left the bloc, could take part in a new EU defence pact, known as Permanent Structured Cooperation (PESCO), which aims to help the EU fund, develop and deploy armed forces together. It will not amount to an EU military. The breakthrough of an impasse which dates back to June 2018 also helps to revive EU defence integration efforts launched with great fanfare in December 2017, in part to show unity after Brexit, but which ran into bureaucratic hurdles. In May this year, the European Commission made a long-awaited proposal to earmark 8 billion euros of its next budget on a new, complementary EU defence fund, keeping alive a Franco-German desire to deepen military cooperation among EU nations that have long pursued independent projects. While the United States, the world’s biggest military power, has 30 weapons systems, the EU has 178. The bloc has 17 types of battle tank, compared to just one in the United States. “The rules on participation of non-members was the last missing piece of the puzzle,” a senior EU diplomat said of Wednesday’s decision, which will allow Britain and others, such as Norway and the United States, to take part in future projects to develop aircraft, helicopters and weapons.