Motorists could be facing an additional tax if they choose to drive through Melbourne’s CBD.

The Grattan Institute is suggesting a charge to be rolled out right through the CBD within the next 5 years.

Grattan’s modelling found the charge to motorists during peak hours would see the number of cars in the CBD drop by up to 40 per cent, and speeds improve by up to 14 per cent in the centre of the city.

The state government says they have no plans to introduce a tax whatsoever.

“We have no plans and do not support a congestion tax,” a government spokeswoman told The Age.