The launch of a controversial bus gate in Glasgow helped the council boost its bus lane revenue by more than £700,000 last year.

Glasgow City Council earned £4 million from the fixed penalty notices in 2014/15, up 22 per cent on the previous year's income of £3.3m and almost matching the £4.1m generated by the cameras' first year of operation.

The surge in income is mainly due to the introduction of a new city centre bus gate at Nelson Mandela Place last summer.

The month before the bus gate went live, the number of penalty notices being issued had dwindled to its lowest ever level with just 6,215 fines handed out to drivers in June 2014.

A month later that had soared by 250 per cent to 21,454 fines, with a further 20,735 issued in August.

Motorists snapped driving in bus lanes or through bus gates face an automatic £60 penalty, reduced to £30 if paid within 14 days.

By September, fines had dipped back below 10,000 after the council was forced to improve the bus gate signage amid complaints from motorists that they had been unable to spot the road markings in time to avoid it.

The figures, obtained under freedom of information, come three years after Glasgow became the first city in Scotland to deploy the cameras in April 2012.

They have proven divisive, with the council and bus operators arguing that they cut congestion and encourage more people onto public transport and critics blasting them as a "cash cow".

However, there is evidence that motorists are altering their behaviour, with the number of fines down substantially despite the number of cameras in use reaching an all-time high in March.

In May 2012, the first full month of operation when ten cameras were active, 17,579 penalty notices were issued.

By March this year, with 16 cameras, the number of fines issued was down to 9,819.

Neil Greig, Scotland-based director of policy for the Institute of Advanced Motorists (IAM), said: "It's good news that the number of tickets issued are going down as that shows the signposting has improved and motorists now understand the system better. But just short of 10,000 a month is still far too many and shows the Council must work harder to ensure no one is any doubt about operational times or the extent of the bus lane regulations.

"The really positive statistics are still missing however. What were all these millions of pounds of fine income spent on to benefit road users? Have the buses been running faster and more predictably as a result of the cameras? Have more drivers switched to the bus? Unless the council can answer these questions the accusation that it is all about raising money will not go away."

A spokesman for the Confederation of Passenger Transport Scotland, which represents operators, said bus lanes "improve bus reliability, encourage modal shift, reduce traffic congestion and lower vehicle emissions".

He added: "When road space is at a premium, as it is in Glasgow city centre, it makes sense to give priority to those vehicles that can move the most people in the most efficient way."

A spokeswoman for Glasgow City Council said: "The number of people using bus lanes has steadily decreased since the introduction of enforcement cameras in 2012, demonstrating that drivers are changing their behaviour and that public transport is getting priority on some of the key routes into and across the city.

"Camera enforcement is an effective deterrent and offences have fallen however driving in bus lanes has always been an offence for anyone but authorised vehicles and if a driver is found to be illegally driving in a bus lane a charge will be issued."