Labour health spokeswoman Jackie Baillie said the target of having 98% of all patients in A&E units treated in four hours has been dropped in favour of a 95% target.
She attacked Health Secretary Alex Neil on the issue after figures last week revealed there were 323 cases in December where patients in A&E waited more than 12 hours.
National standards set out that at least 98% of people in A&E should be either admitted or transferred for treatment, or discharged from hospital, within four hours. The figures show that this was achieved for just 90.3% of patients in December.
Ms Baillie hit out at the "cut in the waiting times target", saying: "Instead of 98% he has cut the target to 95%.
"This admission of failure was sneaked out. They moved the goalposts. They changed A&E waiting times in a spectacular sleight of hand. This is a sneaky and dishonest approach which is more about spin than it is about patient care and, frankly, the people of Scotland deserve much better."
Mr Neil said a new interim target of 95% was brought in for A&E patients. But he insisted Labour's claims are wrong and that the 98% target is still in place.
"Scotland is the only country in the UK which still has the higher 98% standard as the UK Government dropped the 98% standard in June 2010," the Health Secretary said.
"We believe that the performance of NHS boards in A&E must improve which is why we've put in place the £50 million unscheduled-care action plan. This includes a new interim target that 95% of A&E attendances must been treated within four hours by September 2014, as a planned step towards all NHS boards meeting the more rigorous 98% standard."
Tory deputy leader and health spokesman Jackson Carlow also criticised the Government.
"It's incredible that the Scottish Government's response to this crisis is to bring in a reduced target through the back door," he said.
"This is the SNP admitting it has failed on A&E waiting times. Instead of lowering standards and expectations, the Scottish Government should be doing the opposite."
Ms Baillie pressed the Health Secretary on A&E waiting times in a Holyrood debate.
"It is a common occurrence to have old and very ill people lined up in corridors and trolleys because there are not enough beds. In some cases these are the lucky ones. Others don't make it out of the waiting room because there are no trolleys for staff to put people on," she said.
The Royal College of Nursing has complained of there being "not enough staff, or beds or resources, within the system", she said.
"It ill-beholds the cabinet secretary to ignore what doctors and nurses are telling him. You can't ignore what you've been told. The system is at the point of breaking."
But Mr Neil claimed Labour has a "real cheek" to complain about A&E waiting times because when in power in Scotland it put forward plans to close the accident and emergency units in both Ayr and Monklands hospitals.
If it had done this, "A&E waiting times today would have been much, much worse than the problems we have had over the winter".
The previous Scottish Executive did not measure the performance of A&E departments against a target because it was too afraid to do this, he said.
Mr Neil continued his attack Labour's record on the NHS.
"Prior to this Government coming to power, patients were waiting six months for an outpatient consultation following a referral; six-month wait for inpatient and day case treatment."
During the Labour-led Scottish Executive, tens of thousands of patients were put on hidden waiting lists and "left to languish for treatment", he said.
"They don't have the moral right to criticise anybody because their performance was dreadful.
"Under this Government waiting times are now significantly better and amongst the best in Europe."
The SNP administration is "committed to patients being treated more quickly, and more patients are being covered by our waiting times standards".
He said: "Let us get things in perspective. Of course there are problems in the National Health Service. But the National Health Service this year will spend £12 billion; six million times this year someone in the National Health service will see a doctor; over 1.5 million people will be referred to accident and emergency."
Ms Baillie argued that, under the SNP, increasing numbers of patients are being classed as unavailable for treatment, excluding them from waiting times targets.
There have been "sharp rises" in the number of patients not available for treatment.
"Former cabinet secretary Nicola Sturgeon knew about it but chose to ignore those warning signs."
Public spending watchdog Audit Scotland highlighted the increasing number of patients being marked as unavailable for treatment to the Government in both 2010 and 2011, she said.
"Despite the evidence before their very eyes, they did absolutely nothing. Not one single iota of action was taken. And the consequence: thousands of patients, thousands of them, have been let down by the SNP, parked on hidden waiting lists."
The Nationalists have "put an obsession with targets ahead of patient care".
Conservative MSP Nanette Milne said recent events demonstrated that "clinical priorities can be skewed to achieve centrally driven targets", as she warned against politicising waiting times.
"A key focus in recent years has been on waiting times with increasingly demanding targets set by government, which have been met only because of very hard work by a conscientious and dedicated NHS workforce who are under constant pressure from management, who have to answer in turn to their political masters.
"I hope that lessons will be learned from recent sorry events and that Audit Scotland's recommendations to Government, to NHS boards, to their non-executive directors, and to ISD Scotland will be carried out in the interests of both patients and NHS staff."
Liberal Democrat Jim Hume said: "Rather than focusing on getting patients in and out of A&E units faster, the SNP accepted defeat and dropped the A&E treatment target. But SNP ministers should be clear that there can be no shortcuts on patient care.
"Our hard-working NHS staff and patients have been let down by cowboy tactics. What we need from the Government now is a commitment in our NHS which seeks to improve the quality of care."
According to the SNP's Bob Dorris, Labour is using the NHS as a "political football to seek party advantage".
He said: "I would hope that we could unite across party boundaries wherever possible to support our NHS. However, I believe that the current Labour strategy only seeks to create conflict quite deliberately."
Labour's Neil Findlay said concerns raised by Labour "more often than not" come from NHS staff trying to cope within the system.
"As politicians, we should know what is happening in the NHS in our regions and our constituencies. We have caseloads that reflect the realities of the cuts to budgets, the reductions in staffing levels and the lack of resources and falling morale."