The existing road bridge is said to be close to capacity

Supporters said a new road crossing would help reduce congestion and stimulate the local economy.

Mark Hood, from Fife, has started the bid for funding by launching a petition at the Scottish Parliament, supported by Labour MSP Helen Eadie.

However, environmental campaigners have opposed the idea, claiming it would undermine efforts to tackle congestion and cut climate pollution.

An independent report said in September that the current Forth Road Bridge was running close to capacity and could fail to cope with traffic levels within the next six years.

The executive must make the building of a new bridge a priority Mark Hood

It backed the idea of constructing a new crossing, which could cost more than �700m.

The petition urges the Scottish Executive to consider the need for a new bridge.

Increases in population, particularly in Fife, and more freight being transported by road, had led to the existing bridge carrying three times its original intended load, Mrs Eadie said.

"There is an urgent need to press ahead with plans for a new bridge," said the MSP.

Mr Hood said: "For the foreseeable future we will need a Forth Road Bridge capable of carrying a high volume of car traffic across the Forth.

"The executive must make the building of a new bridge a priority."

Economic benefits

Supporters have argued for the economic benefits of the project, particularly for Fife.

Last month, bridge operators Forth Estuary Transport Authority voted for the introduction of a variable tolls system to ease congestion and pay for repairs.

The plan must first have ministerial approval before a public consultation can begin next year.

Friends of the Earth Scotland supported the tolls proposal but said demands for a second bridge were premature.

Scottish Green MSP Mark Ruskell said: "A second Forth road bridge would eat up all the funding for public transport improvements needed to give people a real alternative to the car.

"Doubling the capacity for road traffic to cross the Forth will in time double the congestion and double the pollution - its time to make the break with spiralling traffic growth and put serious investment into the alternatives."