The British state will no longer be able adequately to protect the public from criminals and the growing threat of homegrown terrorists if the Conservatives push through their plans to cut further into police numbers, the outgoing leader of the country’s chief constables has told the Observer.

Sir Hugh Orde, the retiring president of the Association of Chief Police Officers (Acpo), the body responsible for the strategic coordination of police forces in times of national emergency, claimed that under Tory plans to take another 20% from the police budget on re-election, the risk to citizens would rise “exponentially”.

The coalition has already slashed the police budget by around 26% over the last five years, at a cost of 35,000 officers, and has signalled its intention to maintain that rate of cuts.

New figures from the House of Commons library show that if Tory spending cuts announced in last year’s autumn statement – which would lead to public spending falling to just 35% of GDP – were applied equally across un-ringfenced departments, they would lead to the loss of 29,900 police officers and 6,700 community support officers by 2019/20, bringing the ratio of officers to population to its lowest level since records began.

On Saturday it emerged that forces were already drafting radical plans to deal with next year’s cuts.

In the face of the significant growing threats from terrorism and cybercrime, Orde admitted he now had doubts that the state would any longer be able to fulfil its fundamental obligation to keep the public safe if further policing cuts of the size proposed were made.

He said that wider cuts to the public services had already put an increased burden on the police as a “service of last resort”, with officers having to fill in for ambulance crews and deal with those let down by depleted mental health care.

Orde, who said he had concerns about the future of community policing, said: “The police force is shrinking and the population is growing. And the diversity and the complexity of the population is growing at the same time.

“The notion that you can take money out of policing and numbers out of policing without increasing the risk exponentially is flawed. The question is, where is the tipping point? My sense is that it is getting very close.

“The bit I struggle with is when spending on police is 0.89% of public expenditure, if you take 26% out of that, and then another 20% out, there is a question about the state’s responsibility for citizens’ security. I think that will need to be seriously considered by whoever comes to power.”

Asked whether the police would have sufficient resources to adequately protect the public in the face of such cuts, Orde, who left Acpo two weeks ago, said: “In my professional judgment the answer is no.

“I think there are ways we can minimise the impact, some of which requires political leadership that is absent. The force structure is now fundamentally flawed: 44 police forces is far too many. Collaboration, which is the current government’s solution, is important, but it is faintly ridiculous to think that all this huge bureaucracy of getting agreement across forces, and across police and crime commissioners, will deliver a consistent and efficient police service. It is just not going to happen.

“The last defence the home secretary mounted of why she wouldn’t [merge forces] was at the superintendents’ conference last year which was a hopeless defence.

“In Scotland, Sir Stephen House is one chief constable, he has amalgamated eight forces. He is still delivering, crime is at an all-time low, their confidence is at an all-time high. It is a bizarre obsession with cap badges.”

Orde, who took over at Acpo after seven years as chief constable of the Police Service of Northern Ireland, said he was particularly concerned at how a depleted unreformed police force would be able to deal with “disorganised terrorism”.

He said: “No one is saying policing should be exempt from cuts and we have proved we can deliver. The question is how much further can one go.

“If one looks at the emerging threats to the citizens: the disorganised, almost franchisee terrorist, who decides today to be a member of al-Qaida who goes on the internet to create some device and plant it in a shopping centre, is a very different animal to deal with. Dying was never part of the plan with the IRA. Which makes it a completely different threat that requires huge resource.

“The critical element, it seems to me, with dealing with people who will start to behave differently in their communities is the confidence in that community to speak to the local cops. If the cops aren’t there and that relationship has not been built we won’t get the intelligence.”

Orde admitted that relations between the police and the home secretary, Theresa May, had been strained in recent years. He claimed that tensions about the tactics used during the 2011 riots had exacerbated the situation, with ministers questioning why baton rounds (plastic bullets) were not being used on rioters.

He said: “To use excessive force in what was a unique experience even in our history, to overreact by using excessive force, will cost the service for the next three, four, five generations. Baton rounds were raised [by ministers]. If you look at the YouGov polls most citizens would happily have us shoot them. With leadership, the role is sometimes not doing what the citizen wants and therefore what their representatives want.”