Labour has accused the government of panic measures after a leaked NHS document revealed plans to lengthen ambulance response times for seriously ill patients.

The leaked document, drawn up by the Association of Ambulance Chief Executives and dated 16 December, includes a proposal to change the response time for some “Red 2” patients – those with “serious but not the most life-threatening” conditions – from eight to 19 minutes in England.

The plans come as ambulance services struggle to cope with increased demand, reflecting pressures across the whole NHS in general, but particularly now in winter with illnesses such as flu and norovirus on the rise.

“This looks like a panic measure,” Andy Burnham, the shadow health secretary, told Sky News. “The government is rewriting the rules and moving the goal posts during the winter… without proper debate or proper consultation.”

Unions were also quick to raise concerns, demanding to see the clinical evidence for changes, which they said could put lives at risk.

“What they are doing is highly dangerous – instead of admitting that there is a serious problem in ambulance and emergency care and putting solutions in place,” said Rachael Maskell, head of health at the Unite union, who argued that cuts in social care and other parts of the NHS, along with fragmentation and privatisation have put increased pressure on emergency services.

The NHS says pressures on ambulance service reflect pressures across the whole system.

“Staff at ambulance trusts are already working at a high level of intensity treating a greater number of urgent and emergency patients year on year, providing high quality and safe care. We know the number of people visiting A&E is increasing as there have been 550,000 more A&E attendances overall so far in 2014 compared to the same number of weeks in 2013,” said Heather Strawbridge, chair of the south western ambulance service.

She pointed out that pressures on ambulance services are growing each year – 25% over past five years – and ambulance trusts are working differently, together with other NHS providers, to maintain high quality care.

Clifford Mann, president of the College of Emergency Medicine, said the proposals were in large measure a pragmatic response to an enormous number of responses which did not really correspond to that much of an increase in very severe illnesses and injury. But he criticised the way the proposals had emerged.

“The first I have heard of them is today. I think that any change, for it to be credible, needs to be tested,” he told BBC Radio 4’s The World this Weekend. “I would have thought the sensible thing to do is to pilot this either for a period of time or in a number of different areas and to be able to produce data which compares with the same area or the same period last year.”

In an indication of the pressure on ambulances, London Ambulance Service last week asked for help from paramedic teams from the rest of the country because of increased demand. The appeal followed its busiest ever week with 11,008 call outs for the most seriously ill and injured. Despite increased demand, LAS has more than 400 vacancies as it struggles to recruit people. As demand surges, LAS is missing response targets.

Under the current system, calls are categorised as Red 1 or Red 2. Red 1 covers life threatening illnesses such as heart attack, trauma, serious bleeding. The national target is 75% of all Red 1 patients must be reached in eight minutes and 95% must be reached within 19 minutes. Red 2 targets are currently the same, except that the “clock start” can be up to 60 seconds after a call is connected.

The changes proposed in the leaked document are to move a “small number” of Red 2 categories - those where a short extra wait “could have a potentially serious detrimental impact” - to Red 1. The other proposal – and this is where concerns have arisen – is to give ambulance services more time to assess Red 2 calls.

Under the proposed changes, just under half of Red 2 calls will keep the 75% within the eight-minute target, but trusts will have up to three minutes from receiving a call before the clock starts. It means ambulances will have a total of 11 minutes to reach a Red 2 patient rather than nine. It is also proposed that about 40% – instead of the current 95% – of Red 2 calls will have a 19-minute response target.

A Department of Health spokesperson said: “This government has absolutely no plans to double ambulance wait times. We have given ambulances an extra £50m this winter to ensure the service remains sustainable and the secretary of state agreed that NHS England should investigate a proposal from the ambulance services themselves to see whether the service they offer the public could be improved.

“No decisions have been made, and the secretary of state would only agree to proposed changes that improve response times for urgent cases.”