But the average successful bid soared to 91,898 yuan last month despite the regulation and three other measures to combat speculators.

Chen Ji, 28, said she won a plate at March's auction by ignoring the system's "stupid reminder" to offer a rational bid, because "there's no way that I could have won the plate if I had followed that reminder".

Shanghai will also increase the number of car plates offered at the upcoming auction by 2,000, to 11,000.

A new regulation, which states that secondhand cars with Shanghai plates cannot be resold within a year, will take effect on April 20 to help curb surging plate prices.

A similar policy was launched in July and extended the period in which motorists must keep license plates for new cars before reselling them from one to three years.

But such a regulation didn't work because "it has loopholes that scalpers can take advantage of", according to Zhou Wenchao, a salesman at a Shanghai car dealership.

"A plate cannot be resold when it's attached to a new car, but scalpers can arrange for it to go on a secondhand car, making a resale possible," said Zhou.

On the policy of reserving new car plates for new cars only, Zhou said there is concern that the price for secondhand plates will rise, as owners of secondhand cars will be forced to resort to the black market.

But Lin Huaibin, auto analyst with HIS Automotive, disagreed and had faith in the new policy of reserving new- car plates for new cars.