NCP, which owns 580 car parks across the UK, could be sold after a strategic review by Cinven, the private-equity group that owns the business.

Citigroup, the investment bank, has been asked by Cinven to advise on a possible sale or refinancing, according to executives close to NCP.

No final decision has been made but, if sold, the business is likely to be worth in the region of £500m.

Although best known for its car parks, with their yellow-and-black neon signs, the company has diversified into new, more lucrative areas under chief executive Bob Macnaughton.

NCP now has contracts with a number of local authorities, including the London borough of Lewisham and Durham county council, to enforce and run on-street parking restrictions. The company employs the uniformed traffic wardens who ticket illegally parked cars, collect fines and run residential parking schemes.

Cinven bought NCP for £820m in 2002. However, having refinanced the business in 2003, following a £600m sale and leaseback of more than 100 car parks, bankers believe the private-equity firm will more than double its original investment if it sells the company.

NCP still owns a property portfolio of 170 freehold or long-leasehold car parks, which are thought to be worth nearly £400m.

It also has the contract to collect unpaid fines from drivers who have attempted to evade charges for the London congestion zone, and has the power to clamp or seize cars belonging to people who have not paid fines.

However, its central London car parks have been hit hard by the introduction of the congestion charge, which has reduced the number of people driving into the capital.

Last year NCP courted controversy when it ran a competition in Westminster to find the warden who issued the most parking fines. The winner would have won a brand-new Vauxhall Corsa.

Ronald Hobson and Sir Donald Gosling, the founders of NCP, began by buying up derelict bomb sites in London after the second world war.

Having worked together for 52 years, they sold NCP to Cendant, a US company, in 1998 for £805m, earning them £580m. Last year Gosling, 76, threw a £100,000 party in Cap d’Antibes for his mother’s 100th birthday.

As well as owning sites across the country, from Aberdeen to Exeter, NCP runs car parks at 10 UK airports, including Gatwick and Heathrow.

The firm also manages car parks at Euston, King’s Cross, Paddington, Victoria, Waterloo, Manchester Piccadilly and Edinburgh Waverley stations on behalf of Network Rail.

In addition, the company has a contract to run 25 stations on behalf of London Underground.

In 2004, turnover at NCP rose to £400m and it reported earnings before interest, tax, depreciation and amortisation of £40m.

Cinven is one of the largest private-equity businesses in Europe. In Britain, the firm owns Fitness First, the health- club chain, and a stake in Gala, the bingo-and-casino group that is drawing up plans to float.