Correction: Is to resign. He is expected to make a statement at 13:00.
It looks like he wants to fight for his seat on a anti-42 day detention footing.
From the BBC:
It is thought he wants to trigger a by-election in his Haltemprice and Howden seat.
Mr Davis has been a passionate opponent of plans to extend the terror detention limit to 42 days.
It is thought he has privately threatened to resign if the Tories wavered on the issue. He will make a statement shortly.
UPDATE: I've just listened to David Davis make his statement to the press. All I can say is BRAVO David. It takes courage to stand on principle and put his political career at risk. I know this will cause some division in the party, but as far as I'm concerned David Davis has just announced that he will fight to protect my Liberty. He should receive the support of all free men and women.
Very well done David.