Target times for ambulances to reach some seriously ill patients could be lengthened, the BBC has learned.
A leaked NHS document includes plans 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.
It said the plans had been backed by Health Secretary Jeremy Hunt, subject to approval by ambulance trust bosses.
The government said no decisions had been made. Labour has demanded answers.
And the party has rejected suggestions from ambulance chiefs that it was told about the plan to change response times and raised no objections.
"We have never given any support for this plan. When raised in passing, [Shadow health secretary Andy] Burnham explicitly warned of the need for caution and consultation as he repeated today, " a Labour spokesperson told BBC political correspondent Carole Walker.
"The evidence needed to be produced first and it hasn't been. Instead, ministers are forcing it through from January, in the middle of a crisis, without proper planning," the spokesperson added.
Mr Burnham has written to Mr Hunt asking him to explain why the measures - proposed to be brought in within weeks - were not disclosed to Parliament days after he signed them off.
The leaked document, drawn up for the Association of Ambulance Chief Executives and dated 16 December, said NHS England had "explicitly stressed" the plans were confidential and "should not be disseminated beyond the group" involved in the discussions.
The document said there were existing plans for changes "after the general election" in May.
But it said Prof Keith Willett, head of acute care at NHS England, had made an "urgent request" for discussions due to "unprecedented demand" on health services - and the "target for implementing these changes was the first week of January 2015".
In an interview with the BBC, Prof Willett stressed nothing had been agreed but the proposals must be taken "very seriously".
He said the plans would be scrutinised, and only implemented if proved safe and following testing.
By BBC home affairs correspondent Sally Chidzoy
A whistleblower leaked the memo to the BBC because of serious concerns over patient safety, fears the plans were rushed at the height of winter when the service is under unprecedented pressure and anger over the secrecy involved.
To many in the service, the general ideas are good - but there this concern it should have been a more thoughtful exercise where time was taken to consult widely and the public was involved in the process.
Paramedics say response times distort their ability to treat patients because they have to chase the clock.
They say some illnesses such as strokes, should be moved up a category.
The target for these changes was early January, according to the document, but it seems unlikely the proposals will now go ahead by then.
One ambulance service director, who asked not to be named, told the BBC: "This is being done for political expediency rather than patient safety and it's being done with the full blessing of Jeremy Hunt.
"This is being pushed through with limited consultation with the chief executives and the health service as a whole."
Martin Berry, executive officer of the College of Paramedics, said he was not opposed to change but it could not be done "behind closed doors".
"There's been no effort to engage with the paramedic profession. We're just very concerned about the way this has been kept in the dark."
The College, which represents paramedics across the UK, said it would be discussing the proposals with AACE and NHS England and seeking to be involved in negotiations.
President of the College of Emergency Medicine, Dr Clifford Mann, said the proposals appeared "sensible" but they needed to be trialled to provide evidence to prove it was not a "cosmetic exercise".
- Red 1: Respiratory or cardiac arrest - response in eight minutes
- Red 2: All other life-threatening emergencies, such as stroke and fits - response in eight minutes
- Other response times are agreed locally
Ambulance trusts dealt with almost 8.5 million emergency calls in 2013-14 in England, an average of 16.1 calls per minute.
The national target is for ambulance trusts to reach 75% of Red 1 patients within eight minutes, and 95% within 19 minutes. The time starts as soon as an emergency call is connected.
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 to Red 2 are:
- A "small number" moved to Red 1 - those where a short extra wait "could have a potentially serious detrimental impact"
- Just under half to keep the 75% within eight minutes target, but trusts will have up to three minutes from receiving a call before the clock starts
- About 40% to have a 19-minute response target, as well as three minutes before the clock must start
The Red 2 category includes strokes and fits, but the document does not say which conditions would be put in each of the new categories.
It said the proposed changes could bring "substantial improvements".
The document also said trusts would be able to cut the number of fast-response cars being used in favour of deploying more double-crewed ambulances.
But it acknowledged the plans have not had the "breadth of exposure that would normally be expected".
Mr Burnham said: "Jeremy Hunt was dragged before Parliament last Thursday to answer questions on NHS winter planning but treated [it] with contempt. It is outrageous that he decided to keep MPs and the public in the dark about a decision he had already taken and one which will have far-reaching implications across the NHS...
"This leak leaves Jeremy Hunt with extremely serious questions to answer. He must do so today."
He said if the health secretary did not have an "acceptable reason for withholding information" he should make a full apology to MPs.
Mr Hunt has not commented but the Department of Health said the health secretary "would only agree to proposed changes that improve response times for urgent cases".
The leak comes after it emerged on Friday that pressures in England's A&E units had hit record levels, with the lowest percentage of patients seen within four hours since monitoring began in 2010.