London's businesses have given the congestion charge their approval as it reached its six-month anniversary.

In a survey of 500 companies, 49 per cent believed the scheme to be working, compared to 16 per cent who said it was not and 35 per cent who were undecided.

While traffic levels in the centre of the capital have reportedly fallen by a fifth, 71 per cent of businesses claimed they were yet to see an effect on their profits.

Only two per cent of business owners claimed to be considering leaving London.

In contrast, 34 per cent said the £5-a-day charge had enhanced the city's image.

"The results demonstrate that support for congestion charging among the business community remains firm," said Julia Lalla-Maharajh from London First, the business membership organisation which commissioned the poll.

"What is clear, however, is that businesses feel they do not know enough about where the revenue is going, while one fifth felt that the technology was inadequate."

The congestion charge recently came under fire, after Transport for London announced last month that it had been "too successful", leaving a £64 million budget shortfall.

As a result, some transport projects have had to be postponed, while the private company running the scheme has been handed £31 million from the budget.

Meanwhile, an ongoing study is examining a possible extension of the scheme further into Westminster, Kensington and Chelsea, something motoring groups have warned against.

"It's way too premature to consider expanding the zone, when the jury is still out on the overall success of the scheme in central London," said Paul Watters of the AA Motoring Trust.

However, Liberal Democrats have called for the scheme to be adopted in other parts of the country.

"In just six months, congestion charging in central London has already proved a success," said the party's transport spokesman Don Foster.

"What a pity the government didn't have faith in the scheme earlier. Now ministers must work with councils across Britain to identify congestion hotspots and introduce similar schemes.

"Elsewhere, as in London, money raised from congestion charging must be used to boost public transport alternatives."