South Wales Police warned of an effect on routine traffic policing and major events
A chief constable has warned her force will stop responding to accidents on the M4 from April - unless dedicated traffic officers are introduced.
Barbara Wilding said South Wales Police faced a £14m "worst case scenario" funding shortfall and could no longer afford routine traffic patrols.
Ms Wilding also said that without more funds she would have to charge major event promoters full policing costs.
These would include pop concerts, football and rugby matches.
Royal visits could also be scaled back.
Ms Wilding said with precept funding, raised from the council tax, the force was facing a "worst case scenario" of a £14m funding deficit.
Speaking to BBC Wales, Ms Wilding said she had been telling senior assembly government ministers for two years that the force would be facing cuts in the services it delivers without more money, but to no avail.
The assembly government warned that police authorities had to be efficient and the minister did not expect "unreasonable council tax increases" to be imposed on the "already hard-pressed householder".
Ms Wilding said the work of dealing with accidents, directing traffic and clearing debris should either be done by new traffic officers from the Highways Agency, or be funded through extra money for policing.
"We will be building a business case to the Welsh Assembly Government to say if you want us to carry on policing the motorway, responding to those issues, then you must pay for it," said Ms Wilding.
"If you don't pay for it, we will no longer be doing it."
Chief constable Barbara Wilding says South Wales Police will have to stop doing certain things
Later in a statement, she said police would still attend collisions but not routine traffic incidents - which were already dealt with by Highways Agency officials in England.
"I would ask the public a simple question, would they rather my officers pursue the culprits of a burglary or sat in their motorway patrol car waiting for a truck to have its flat tyre changed?
"It's about using our specialist resources in the most appropriate way. For every hour the M4 is closed, it costs the public of south Wales, £1m."
Ms Wilding also said that without funding increases, she would be forced to charge promoters of major events in Cardiff, such as rugby matches and pop concerts, the full policing costs.
She also suggested the number of events during royal visits to south Wales may have to be scaled back due to pressures on her budget.
"We will start charging for the major events, we will have to, because at the moment, our policing levels are going down to 1974 figures," she said.
Ms Wilding gave an example of the recent Cardiff versus Swansea football match, which she said cost £250,000 to police, for which South Wales Police received only £25,000 from the event promoters.
The chief constable also called for South Wales Police to be paid an extra £1m by the assembly government in recognition of the costs of policing the capital city of Wales, as happens in Scotland.
Police on duty at the Cardiff v Arsenal FA Cup tie in January
"The implications are quite simple really, that if football clubs or people putting on events with pop stars are not able to pay for the level of policing that we say is needed for that event, on risk - it's all done on risk - then they may have to consider whether they can run the event in the first place," she said.
"And that, of course, has a huge impact on the economy of hotels, shops, transport, the whole thing."
At a meeting of the South Wales Police Authority, Ms Wilding also urged members to agree a 9.8% increase in the amount council tax payers are charged for policing so patrols of the M4 could continue.
The Welsh Assembly Government said police funding was a matter for the Home Office, with South Wales and Gwent police forces legally responsible for policing the M4 in south Wales.
A spokesman said a transfer of certain powers to highways agencies to improve efficiency of road policing and to free up officers for law enforcement had already been proposed.
The trial of a rapid response system for the M4 and A55 was also subject to discussion, while two traffic management centres for the same roads had been funded.
On the policing of major events, the spokesman said South Wales Police Authority was funded by a formula based on need, which took into account special events.
"All local authorities including police authorities should manage their resources as effectively and efficiently as possible," said the spokesman.
"The minister will monitor carefully budget setting in the coming weeks and does not expect authorities to seek unreasonable council tax increases from the already hard pressed householder.
"The minister will consider using the capping powers available to him in the event of excessive budget increases".
The Liberal Democrat local government spokesman Peter Black said the assembly government should meet Ms Wilding "half way" to resolve the issue.
"Both the Home Office and the assembly government are underfunding our local police forces," he said.
South Wales Central AM Chris Franks criticised Ms Wilding's threat to stop policing the M4 as "frankly astonishing".
'Free up the police'
The Plaid Cymru AM added: "It might be a good headline grabbing comment but is it responsible for the most senior police officer in south Wales to do that?
"The reluctance of councillors on the police authority to accept demands for a 9.8% council tax rise might disappoint the chief constable. However, to lash out in such an intemperate manner is very unhelpful."
Meanwhile, Ms Wilding's calls for dedicated traffic officers were echoed by the assembly's enterprise and learning committee.
Committee chairman Gareth Jones AM said Wales' two major corridors in its road network - the A55, and the M4 in the south - were vulnerable to congestion.
Any such jams went onto have "worrying social and economic impacts", he said.
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