AMSTERDAM (Reuters) - Activist investor CIAM said on Tuesday it has asked the management boards of Dutch-Belgian supermarket operator Ahold Delhaize to convene an extraordinary meeting of shareholders to vote on the company's takeover defence mechanism.

At its shareholders' meeting on April 11, Ahold's boards argued that the defence, which had been approved by shareholders for a 15-year period ending in 2018, can now be extended unilaterally and indefinitely.

A group of institutional shareholders led by CIAM disagree and may mount a legal challenge.

(Reporting by Toby Sterling, editing by Louise Heavens)