The PM said the he had no plans to legislate, although he hinted ministers would not stand in the way of MPs attempting to introduce a backbench Bill.
Meanwhile, Home Secretary Theresa May echoed Culture Secretary Maria Miller earlier this week in suggesting a more limited reduction to 20 weeks.
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The latest weather update for the east with Philippa
The latest weather update for the west with Philippa
Philippa has Saturday's weather for the east