Dynegy's board has accepted a sweetened $603 million takeover bid by Blackstone Group LP but the deal still faces opposition from a prominent shareholder.

The private equity group on Tuesday increased its offer for the Houston power plant owner by 11 percent to $5 per share.

Shareholders vote on the deal later Wednesday and approval is far from certain. Financier Carl Icahn said Tuesday he would oppose the increased offer, saying it still undervalues the company. Icahn owns a nearly 13 percent stake in Dynegy.

Icahn also said the takeover agreement discouraged other bidders from submitting competing offers. Dynegy has agreed to pay Blackstone a $16.3 million breakup fee if the deal isn't approved.

The shareholder vote is expected later Wednesday in Houston.