U.S. chipmaker Microsemi Corp is challenging a shareholder defense adopted by Zarlink, the Ottawa-based rival it is trying to take over with a hostile bid valued at C$550 million ($561 million).

Zarlink, which has rejected the bid and is seeking higher offers, said on Friday Microsemi was challenging the so-called poison pill takeover defense at the Ontario Securities Commission (OSC), Canada's main securities regulator.

"The rights plan is intended to ensure that Zarlink has time to bring its previously announced process to its conclusion for the benefit of shareholders and debentureholders," Zarlink Chairman Adam Chowaniec said in a statement.

Zarlink's board adopted the poison pill, which is triggered if any one shareholder buys 20 percent or more of the company, on July 25.

The OSC will hear Microsemi's application to challenge the plan on October 3 and 4.

The Microsemi bid expires on September 22 but can be extended.

In Friday's press release, Zarlink reiterated the board's unanimous recommendation that holders of shares and debentures reject the Microsemi offer.

Earlier this month, Zarlink said it expects a white knight to best the Microsemi offer, and that it is in talks with at least 15 potential buyers.

($1=$0.98 Canadian)

(Reporting by Trish Nixon; editing by Peter Galloway)