THE cost of the M74 extension has soared again, to almost £700million bringing the bill to more than £26,000 per foot.

A report published today by spending watchdog Audit Scotland revealed the five-mile, six-lane stretch will now cost £692m, that's £35m more than the price last announced.

The M74's missing link', which is already three years late, is Britain's most expensive road, working out at £26,400 per ft or just over £80,000 a metre.

Audit Scotland blamed poor management for the massive price hike.

The original estimate in 2001 was £245m and the last cost stated, in February this year, was £657m. That was made up of £445m plus an allowance for around £12m to deal with possible mine workings along the route on top of £200m spent.

The study also said inflation in the construction industry and delays caused by a failed legal challenge by environmental campaigners had caused costs to soar.

Transport Scotland said administration costs, buying land and the price of cable and pipe diversions - none of which were included in the original estimate - were also responsible.

The road will complete the motorway network and reduce congestion on the busiest stretch of the M8 through Glasgow. It will also provide links to regeneration projects in the city's East End and will bring much-needed investment in homes and jobs. It is due to be completed in 2011.

The M74 extension is just one of the projects highlighted in the Audit Scotland report which have spiralled above budget.

The study also revealed the cost of the Glasgow Airport Rail Link is also set to rise from the original £210m.

It is now likely to cost between £300 and £400m, although this is due to it being combined with an already planned Network Rail upgrade worth £173m.

The study showed that only two out of five big projects in Scotland - such as new roads, railways, hospitals and prisons - were completed on budget, while many were also late.

Among them was Glasgow's £87m Beatson Oncology Centre, based at Gartnavel, which ended up 30% over budget, including an extra £8m for inflation.

Robert Black, Auditor General for Scotland, said: "Good decisions can be made only if there is accurate and robust information about the likely costs, benefits and timescales of projects.

"There needs to be improvement in the information that is available at the earlier stage."

Conservative MSP Alex Johnstone, Shadow Minister for Transport, said of the M74 issue: "The current SNP Government cannot sit back and relax in the face of the numerous errors made by Labour and the Lib Dems."