LISBON, May 16 (Reuters) - Euro zone inflation is in a sustained decline towards 2% and the European Central Bank's interest rates will certainly start diminishing, ECB policymaker Mario Centeno said on Thursday, declining to predict whether it will happen at the next meeting in June.

"This nearing of the inflation rate towards 2% is real and we are certain that the monetary policy interest rate will come down," he told a news briefing in Lisbon.

"The market expects that the interest rate reduction will begin in June... I'm not going to anticipate the decision," he said, adding that the rate will be decided based on data at each meeting, and that he preferred gradual cuts rather than a sharp, sudden cut. (Reporting by Sergio Goncalves, writing by Andrei Khalip, editing by David Latona)