Chief Constable Sara Thornton said resources were being diverted from mainstream policing in England and Wales, leading to backlogs in police control rooms and slower response times.
So far in 2017 there have been five terror attacks in the UK, killing 36 people.
Four people remain in police custody following last week's explosion at a Tube station in which 30 people were injured.
The number of arrests made for terror-related offences in the UK jumped a record 379, up 68 per cent in the last 12 months up to June this year, according to figures released by the Home Office.
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"The current level of terrorism, added to the strain and stress the force is already under, is causing real problems and is frankly unsustainable."
She said having officer numbers at 1985 levels and crime figures up 10 per cent in the last year created additional pressures.
"After Manchester attack, of the officers and staff that responded, three-quarters were paid for out of mainstream policing."
She said a response would "never be as good as preventing" attacks in the first place.
Metropolitan Police Commissioner Cressida Dick said her force would "have to make hard choices in the future".
Earlier this year, Metropolitan Police Assistant Commissioner Mark Rowley said that the counter-terrorism policing network was not able to operate at "full strength".
He said that other areas of policing would be at "significant" risk if resources had to be diverted.
"We have also protected overall police funding in real terms since 2015 and we are sensitive to the pressures on police forces across the country.
"We are engaging with them on the demands they are currently facing.