Microsoft set the stage for a full-blown battle to overthrow Yahoo’s board of directors on Monday when it declared it was interested in reopening talks to buy all or part of the internet company – but only if a new board was appointed first.

The intervention, less than a month before Yahoo’s annual meeting, gives backing to the flagging efforts of activist investor Carl Icahn to unseat Jerry Yang, Yahoo’s chief executive, and the rest of the board that failed to reach agreement on Microsoft’s earlier takeover offer.