Yahoo has three weeks to accept Microsoft's £22.3bn bid or face a hostile takeover and a management cull.
In a letter to Yahoo's board, Microsoft's chief executive, Steve Ballmer, threatened to take its offer direct to shareholders in three weeks - possibly at a lower rate - if Yahoo's board fails to respond.
"If we [Microsoft] have not concluded an agreement within the next three weeks, we will be compelled to take our case directly to your shareholders, including the initiation of a proxy contest to elect an alternative slate of directors for the Yahoo board."
Microsoft made its offer on the 31 January but Yahoo rejected the offer as undervaluing the business. The bid is seen as part of Microsoft's move to develop its range of online software products. The deadline for the offer is 26 April.