THE cost of upgrading one of Scotland's busiest stations has soared by £6.3million.

Strathclyde Partnership for Transport was originally told the cost of building work at Partick would be £9.7m, with a further £2.6m needed for professional fees and third party costs.

But contractor C Spencer Construction has asked SPT for a further £6.3m, bringing the total cost of construction to £16m and the final bill to £18.6m.

It is the latest problem to hit work at the station, which is two years behind schedule.

SPT has agreed to pay the Lincolnshire based company almost £2m of its new claim but is disputing the remainder.

A spokesman for the transport authority said the extra cost and delay in completing the project was mainly due to ground conditions, which were discovered when work began.

That resulted in work being delayed while the station was redesigned. Further delays occurred when SPT and Network Rail were involved in a wrangle over insurance liability for workers involved in the project.

And the ownership of a small strip of land beside the station concourse further held up the work.

The SPT spokesman said: "When they started digging the foundations they came across things they were not expecting and could not factor in. As a result they had to investigate and redesign, which caused delays.

"We have agreed an interim payment of £1.96m, but are challenging £4.34m to make sure the claims can be substantiated."

Work on the interchange started in 2005 and was due to finish early last year.

Consultants employed by SPT are studying the details of the construction firms claims, but already believe not all can be justified.

A report to go before a meeting of the SPT this week states: "All claims will be vigorously defended.

"The interchange has always been regarded as a technically ambitious project as it involves construction work over the existing 112-year-old Subway tunnels while maintaining operation of the busy Subway, the suburban rail station and the bus terminal."

Around 4m rail and Subway passengers use the station each year, making it the fifth busiest in Scotland after Glasgow Central, Queen Street, Haymarket and Waverley.