The company that runs London's congestion charge scheme today said that its profits had soared by almost a quarter, rising to £121.2m.

Capita Group, which administers the £5 charge on motorists entering the centre of the capital, said that the 23% rise in pre-tax profits before goodwill in the year to December 31 represented a record for the 15th year in succession.

Despite the scale of the profits and the scheme's success, controversy has surrounded the group's operation of the congestion charge and other public contracts.

After the charge was launched, thousands of drivers had difficulty contacting Capita's call centre to pay it, while others were wrongly sent demands for £80 fines although they had paid.

Last month, the government said that it had paid the Home Office criminal records bureau £3.6m after Capita, which runs the bureau, found itself unable to cope with thousands of postal applications for background checks.

Today, Capita called its implementation of the congestion charge "highly successful", citing a 30% reduction in traffic congestion and an 18% cut in traffic entering the charging zone.

It said that the call centre service had improved, with 91% of calls now being answered in less than 30 seconds - compared with the target of 80%.

The company added that 170,000 people had now signed up to use an SMS texting system for paying the charge, with 3.2 million payments made via the service since it began.