Avon and Somerset's Police and Crime Commissioner and Chief Constable have warned the Government they are facing a "tipping point" in funding.
PCC Sue Mountstevens and Chief Constable Andy Marsh say further cuts to budgets would lead to "extremely serious consequences".
Last week the pair wrote to Nick Hurd, the police and fire minister, sending him a copy of a report they had written about the difficulties they face in Avon and Somerset.
In their report, Andy Marsh and Sue Mountstevens state that the force has been recognised as outstanding in efficiency.
Since the Government's first comprehensive spending review in 2010, the force said it had made savings of £65 million and had 655 fewer police officers. It now has to find a further £17 million by 2021/22.
The pair say they face major cuts in spending at the same time the demands on the force are increasing and becoming more complex. They include extremism, organised crime and drugs and abuse.
They add that tackling mental health issues and missing people can be extremely time-consuming.
Andy Marsh says the situation is affecting his workforce, with increased absences linked to mental health and a cap on annual pay awards hitting recruitment.
Sue Mountstevens says enough is enough:
"We have reached the point where enough is enough and policing in Avon and Somerset cannot be stretched any further. With yet another terror attack on the country’s capital, the reverberations felt locally continue to test all those responsible for ensuring the continued safety of our local communities...
"The current situation is unsustainable. We need investment in policing, funding to strengthen our neighbourhood policing teams in order to focus on prevention, tackling terrorism before it happens".