Traffic wardens were ordered to give out a minimum of 306,000 parking tickets a year by a local council, a leaked contract has revealed.
The document, written by Kensington and Chelsea council, ordered parking enforcement firm NSL not to let the number of parking tickets being issued fall below the annual level.
It also set out a minimum quota of 15,000 wheel-clampings per annum.
Both the council and NSL deny any wrongdoing.
The 10-year contract, which started in 2006, ordered: "The number of valid PCNs issued in a year should not fall below 306,000.
"The number of valid clamps fitted should not fall below the agreed benchmark. This is currently set at 15,000 per annum."
The government confirmed the illegality of parking ticket quotas under guidance issued in 2008.
But more than two years later Kensington and Chelsea Council's written contract with NSL still contained minimum targets.
Only in July 2010 did the council formally amend it. Both the council and NSL say their operations have been lawful at all times.
The council said in practice their quota system came to an end when the 2008 guidance was issued.
Transport Minister Norman Baker said: "Our guidance to local authorities is clear. They should not set targets for the number of penalty charge notices.
"Parking charges are a tool to manage demand for road use and setting targets is therefore inherently contradictory.
"Councils must have regard to our guidance on this matter and concerns should be raised with the local government ombudsman if there is evidence of maladministration."
Parking campaigner Barrie Segal said: "It is an absolutely sensational revelation.
"Local authorities and parking enforcers have always claimed targets don't exist.
"This contract demands full investigation by the Minister of Transport - it's a smoking gun."
Last week a judge said he had no doubt that NSL managers felt under pressure from the company to maximise the issuing of parking fines in Kensington and Chelsea.
He was ruling on the case of Hakim Berkani, a traffic warden unlawfully fired because he had refused to comply with the "clandestine quota system".
The judge found a minimum requirement of 10 tickets per working shift led to some parking officers issuing "predatory and dishonest" fines.
Mr Berkani started working for NSL three months before the government's 2008 guidance. He was suspended by the company in October 2010 and then sacked.
In his ruling, the judge said he was fired because of his opposition to the quota system and his union activities, adding that he could deal with the case without having to look at the contract between the council and NSL.
The written contract contained a confidentiality agreement which read: "We hereby agree we shall keep strictly confidential all information supplied to us."
It detailed how the enforcement firm must reimburse the council for "costs, claims, demands, losses or legal costs" arising from any confidentiality breach.
The contract also allowed the council to deduct £53 from NSL for each ticket below the 306,000 target.
Mr Segal, a chartered accountant, said: "The fact there's a penalty if they don't reach a certain figure shows it's a target.
"In commerce you only suffer a penalty if it's a target.
"It's what motorists always suspected and why they've lost all trust in local authorities."
'Unlawful tax farming'
Between 1996 and last year Kensington and Chelsea Council took £500m in revenue from parking enforcement.
Alasdair Seton-Marsden, who helped bring the contract to light, said: "It's now high time for anyone who believes in civilised society to stop... tax farming of motorists.
"It's been obvious for many years that ticketing and clamping quotas existed.
"Why do our elected representatives believe the rule of law applies to everyone but themselves?"
He said unfair ticketing had a wide impact.
"Supermarket delivery vans attract thousands of tickets - what does this do to the price of food?
"A number of utility companies pay more than £1m in parking fines to Kensington and Chelsea alone - what does that do to electricity, gas or telephone prices?"
NSL is the largest parking warden employer in the UK, providing enforcement for more than 60 councils.
Spokesman Alastair Cooper said: "NSL's original contract with Royal Borough of Kensington and Chelsea predates the 2008 guidance which made it illegal to set targets for revenue or Penalty Charge Notices.
"The contract has subsequently been amended to reflect changes in the law and this means NSL are not penalised based on the numbers of PCNs issued or vehicles immobilised.
"Our operation in the borough (as elsewhere in the country) is entirely consistent with the law."
A spokeswoman for the Royal Borough of Kensington and Chelsea said: "We amended our formal contract in July 2010.
"But it doesn't follow that the old contract terms continued in operation until then.
"We changed our enforcement practices in line with the statutory guidance when that guidance came into force.
"Our practices are entirely lawful and always have been."