British screen star Colin Firth insists the ageing process is catching up with him, as he is having to work harder and harder to stay slim.

The 45-year-old Bridget Jones hunk has restricted himself to a rigorous exercise routine, as his days of fast metabolism are over.

He says, "I get regular exercise. I've got to run to stay in this shape.

"When I was about 20 it seemed impossible to get fat."