The Private Finance Initiative has become the treasure trove of public investment: most major programmes to improve hospitals, schools, roads and other public services depend in some measure on private sector funds channelled through PFI schemes.

The government has already signed up to 450 projects worth £20 billion, and many more are in the pipeline: but the whole concept - offering potentially high returns for comparatively low risks - remains highly controversial.

And now one of the leading PFI companies, AMEC has complained that the PFI system is straining under the weight of Whitehall red tape - while other firms are believed to be ready to pull out of future commitments if costs and delays are not reduced.

Amec is involved in eight PFI projects, and are bidding for several more. World at One speaks to AMEC's Finance Director, Stuart Siddall.