BIRMINGHAM, England, Nov 17 - British Prime Minister Tony Blair said on Monday that Britain should keep open the option of joining the euro and the decision should be made in the national economic interest. Blair told the annual Confederation of British Industry in Birmingham that Britain has enjoyed 44 successive quarters of economic growth and stability was now real. "The decision on the single currency should be taken on whether joining would entrench stability or not," he said. "I believe we should keep the option open, plan for membership and make the decision at the time we want on the basis of our national economic interest. Ruling it out serves no one and simply limits our options unnecessarily." Britain said in June that the economic conditions were not yet right for joining the euro but left open the door for revisiting the tests at a later date.//