David Cameron appeared increasingly isolated last night after senior police officers, MPs and even the Conservative Mayor of London united in a call for him to reconsider police cuts in the face of four days of sustained rioting.

In an attempt to regain the political initiative the Prime Minister had declared that a police "fightback" was under way, and that water cannon were being made available at 24 hours' notice. But senior police chiefs said these would be ineffectual and the real question was not whether they could cope with the current disturbances, but whether they would be able to deal with similar civil disturbances in future with thousands fewer officers.

The Mayor of London, Boris Johnson, defied Mr Cameron's position and openly criticised the plan for 20 per cent cuts in police budgets. "The case was always pretty frail and it has been substantially weakened," he said. "This is not a time to think about making substantial cuts in police numbers."

His stance was backed by senior officers. Writing in The Independent today, Sir Hugh Orde, president of the Association of Chief Police Officers, says the position of the police, already facing "20 per cent funding cuts... will get more challenging as the efficiencies get harder to find".

Firefighters, who have also been on the front line of the riots, added to the pressure. Andy Dark, assistant general secretary of the Fire Brigades Union, said: "Fire crews are angry and sickened by the silence on widespread cuts. We are already stretched and struggling and that will only get worse. How does the Government expect us to cope with far fewer frontline crews in the years ahead?"

The Labour leader, Ed Miliband, is expected to raise the issue of police cuts during an emergency debate on the rioting in the House of Commons today and senior Tory backbenchers have expressed alarm that Mr Cameron has positioned himself "on the wrong side of the argument".

"Mr Cameron only has a couple of days to prove he has this under control," said one senior Tory backbencher. "If he can't do that then he is in serious trouble."

Last night a group of London Labour MPs including the party's deputy leader Harriet Harman met the Home Office minister James Brokenshire and called on him to urgently rethink the scale of the cuts. "It is a big priority for Londoners," she said. "The police are much needed. We must not see numbers cut back."

In stronger language than he has so far used, Mr Cameron branded pockets of UK society as sick. In a statement afterthe Cobra meeting, he said police would be given the resources and legal backing to use whatever tactics they felt were necessary. "We will not allow a culture of fear to exist on our streets. Whatever resources the police need they will get, whatever tactics police feel they need to employ, they will have legal backing to do so." But asked about Mr Johnson's call for a rethink on police cuts the Prime Minister appeared dismissive.

Later the Government hinted that council tenants involved in the violence could jeopardise their tenancy. "People need to understand actions have consequences and, if you do happen to have a tenancy which is a council house and you're involved in these crimes and you're convicted – then there will be consequences," said the Housing minister, Grant Shapps.

Police have made more than 1,000 arrests since the unrest began and at least 160 people have been charged. An 11-year-old boy, a primary school worker and a student were among scores of alleged rioters to appear before magistrates. Several courts were expected to sit all night again to cope with the volume of cases.

But there was confusion about the details of the 24-hour contingency plan to provide water cannon outlined by Mr Cameron. One police officer suggested that the proposal was simply "spin" and said that none of the police chiefs involved were likely to ask for them.

Acpo confirmed that the proposals involved the use of up to six water cannon belonging to the Police Service of Northern Ireland. An Acpo spokesman said: "If the vehicles were required then they would come from the PSNI. At present, it is not felt that they would be effective in tackling the current disturbances."The Ulster force emphasised it was unable to provide the trained operators required to man the vehicles.