A proposal to permanently switch on speed cameras along a section of motorway to raise funds for a cash-strapped police force could be futile because the money would go to the Government, not police, No 10 said.
Police and crime commissioner Olly Martins said Bedfordshire Police were in such a desperate financial plight that he may have to resort to turning on the speed traps between junctions 10 and 13 of the M1 all the time to raise funds.
But Downing Street warned that speed cameras should be used to keep road users safe, not as income generators.
Mr Martins said he was considering the drastic move because he was "running out of levers to pull" to keep his force financially afloat.
He said: "Strict enforcement of the speed limit could raise £1 million and to me that's better than losing 25 more police officers."
But Prime Minister David Cameron's official spokeswoman said: "We are very clear that speed cameras should be about safety, not about raising cash.
"The point we would make to those thinking about using them for other means is that it is important to note that revenue generated from speeding offences doesn't go to police forces, it goes to a central fund."
Money from speed camera fines goes to the consolidated fund - the Government's bank account at the Bank Of England - for general expenditure.
It can then be distributed by the Department for Transport through grants.
Mr Martins has outlined drastic plans to raise money for Bedfordshire Police before, telling the home affairs select committee earlier this week he was open to companies such as the airline easyJet advertising on patrol vehicles and was prepared to sell sponsorship of police uniforms.
He blamed the ideas on spending cuts, with more likely in the Chancellor's spending review at the end of the month when Mr Martins expects £20 million to be slashed from his force's £100 million budget.
Such cuts would force a reduction in officer numbers and a recruitment freeze, he said.
The commissioner said Bedfordshire Police are "stretched to the limit", with 169 officers per 100,000 people against a national average of 232, and 388 in London, despite having some of the highest levels of gun and knife crime, burglary and robbery in the country.
He said: "That's why I am prepared to take desperate measures to avoid reducing our already anaemic police numbers."
Mr Martins urged those against the idea of permanent speed cameras and "sponsored" police to sign a petition calling on the Government for increased funding, saying: "We have to make them listen."
The RAC said the majority of motorists see speed cameras primarily as a money-making scheme for the police.
Chief engineer David Bizley said: "It appears that the Bedfordshire police and crime commissioner harbours this view too, or at least he is using this as an opportunity to make a very serious point about resourcing.
"Motorists tell us that they would like to see better enforcement and more roads police officers, but enforcement needs to be prioritised in terms of road safety benefits and not in terms of the value of the revenues generated."