Work will be carried out overnight for the next 12 months

Traffic using the tunnel has increased from four million vehicles a year when it was opened over 40 years ago, to 25 million.

Damp and pollution have affected the structure and the old plastic and aluminium lining has had to be removed.

Over the next 12 months, one side of the tunnel will be closed overnight to traffic while the work is carried out.

Finalising the new design resulted in a 10-month project delay.

The tunnel has been stripped back to its cast iron supports and contractors will replace the old lining with a new fireproof version.

Pink poser

Glasgow City Council's Marco Bardelli said contractors are making good progress.

He said: "We have a steelwork contractor who is putting up a stainless steel support framework and a drip shield which will prevent water dripping into the tunnel.

"Then a mesh will be attached to that, to which a very special fire-resistant concrete lining is sprayed on."

Marco Bardelli said the tunnel will be transformed

It is believed to be the first time concrete has been sprayed onto steel fixed to cast iron.

The council had considered painting the tunnel a pale pink because it has to be easy to clean and light, but instead it opted for green.

The work is coinciding with a replacement for the on-ramp to the Kingston Bridge.

Sue Nicholson, of the RAC Foundation, said it was not ideal to have different roadworks taking place in the city at the same time.

She said: "However, I have to say that because the works are confined to overnight they are not really disturbing peak-time traffic."