When Gordon Brown called the general election that ended his stint in No 10 it was dubbed the “least well-kept secret of recent years”. Yet back in 2010 it blindsided council officials in Birmingham.

Building contractor Amey, one of the biggest in the country, was on the verge of signing a £2.7bn roads deal with Britain’s second city. The problem: it needed central government sign-off. Events in Westminster were in danger of kicking negotiations into the long grass.

As MPs set out on the campaign trail, Amey and the council set about thrashing out the final terms. An agreement with an ominous 666 ­separate clauses was rushed through. With it, insiders say, the foundations of future problems were laid; the devil would be in the detail.

To some, the tale of Birmingham’s roads contract exemplifies much of what has gone wrong with Britain’s use of private finance initiatives (PFI).

The 25-year deal has almost bust Amey, a contractor that employs 20,000 people, and forced its owner, Spanish infrastructure giant Ferrovial, into a fire sale of the company.

“When we acquired Amey there was plenty of problems,” says one senior source at Ferrovial. “Now, the only problem that we have is Birmingham.”

PFI has not always spelt disaster. Launched by Tony Blair in the late Nineties, it was hailed as a magical solution to what New Labour claimed had been years of underinvestment in the country’s public services by the Tories.