Chief Constable Barbara Wilding said the force was facing a funding crisis
South Wales Police's chief constable says she is considering a legal challenge to a decision limiting the force's budget rise to 5%.
Barbara Wilding wanted almost double that figure, and said she was now faced with looking at cutting police jobs.
She said said legal counsel had been chosen to look at the South Wales Police Authority's decision.
In response, the police authority said the decision was "difficult", but it would be implemented.
Ms Wilding said the force had declared a "critical incident" following the authority's refusal to make a big increase in the amount it raises from council tax.
"They did not give us a balanced budget which means we do not have the money to even stand still and do some of the basic things that we need to do such as make sure that we are connected to the police computer network which covers all police forces in the UK," the chief constable said.
She warned she was now faced with potentially cutting jobs.
"Because we haven't got a balanced budget I've got to find the cash from inside the organisation and most of our budget goes on people."
Ms Wilding said up to 140 police officers or 280 police staff could be affected.
She also said the police authority decision had been made behind closed doors and "seemed to come with a condition that I should find another £1m from backroom services and present it to them on the 16th of March.
"Now the decision goes to the very heart of my operational independence and they cannot interfere with that," Ms Wilding said.
Policing on the M4 and at large events could be cut back
She also said she has decided to accelerate a long term plan to merge four of South Wales police divisions into two - merging Swansea and Neath Port Talbot and Bridgend and the Vale of Glamorgan.
She is also looking to see if any police stations or buildings can be closed. She said: "I am not scare-mongering, I am a career detective and used to dealing with facts and evidence - this is the stark reality of the situation".
A spokesperson for the South Wales Police Authority said: "We have been made aware that the chief constable is taking legal advice on last night's special meeting.
"Clearly, the decision arrived at was a difficult one for all concerned.
"Nevertheless, a decision was arrived at and will be implemented, as we are duty bound to notify our constituent local authorities of the precept details by the 1 March," the spokesperson added.
The Police Federation said it was "astonished" about the decision, and felt it was "an abrogation" of its "legal responsibility to provide an efficient police service".
Ms Wilding had earlier warned a shortfall in funding could mean stopping routine M4 patrols and charging promoters of major events full policing costs.
On Wednesday she said the budget rise could also impact on policing golf's Ryder Cup which is taking place in the neighbouring Gwent police force area in 2010.
"It means that the policing may have to be delivered by aid from forces in England because I may very well have to cut some of the areas like my public order units, my specialist search teams, motorcyclists.
"Those sorts of areas that we would normally provide as aid because we have the capacity. we may no longer have the capacity."
The increase in the police precept for South Wales Police, raised through council tax, amounts to an extra 13p per week for a Band D household.
It means the overall policing budget for the South Wales force for the next year will be £248.5m - a 3.4% increase.
Authority members voted against the advice of its officers to set the council tax increase within the capping limits being imposed by the Welsh Assembly Government.
The assembly government had 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".
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