THE inquiry into Edinburgh’s beleaguered trams project has ballooned by £1 million in the last four months alone.

Officials have revealed the total cost of the probe – which is approaching the end of its public hearings – now stands at just short of £8m.

Figures show £6.9m had been spent by September last year, with the majority of the cash going towards staff costs and “professional fees”.

Critics previously accused the inquiry of “spiralling out of control” amid claims the public are losing patience with the process.

It was set up more than three years ago under Lord Hardie, the former lord advocate, but only started hearing oral evidence in September last year.

Costs released at that time under Freedom of Information laws showed a jump of almost £1 million in the space of three weeks as witnesses began giving evidence in person.

Edinburgh’s tram line opened in 2014, three years late, substantially reduced in scope and significantly over budget. It’s final cost will add up to more than £1 billion.

Announcing an inquiry into what went wrong in 2014, then First Minister Alex Salmond promised it would be "swift and thorough".

More than six million documents are being examined as part of the probe, but it will not hold anybody legally responsible or financially liable.