A&E departments are struggling to sustain improvements to waiting times because of NHS deficits, the British Medical Association (BMA) warns.
Staff have been working hard to reduce waiting times in A&E
A BMA survey of doctors blamed a lack of inpatient beds, along with staff shortages and inappropriate demand from people with minor complaints.
A third of doctors surveyed said data was manipulated to ensure government targets were met.
Since 2004, 98% of patients in A&E should be seen within four hours.
Around 500 members of the British Association of Emergency Medicine responded to the BMA survey to investigate the impact of the four-hour target, which has been heralded as a success story by the government.
Almost half of respondents reported that their department was not meeting the target.
And doctors raised concerns that even when targets had been met, departments had needed to use additional funding and agency staff on short term contracts to make sure patients were seen in time.
Government figures show that in 2005-06, 98.2% of patients were seen, diagnosed and treated within four hours of their arrival at A&E, the first full financial year in which this has happened.
However, the BMA survey suggested that data could not be relied upon, due to manipulation of the figures.
The BMA said staff were working increasingly hard to maintain the improvements that had been made to waiting times with the vast majority reporting their workload had increased in the past 12 months.
Almost all respondents said they had experienced direct or indirect pressure to meet the four-hour target.
Mr Don MacKechnie, chairman of the BMA's Emergency Medicine Committee said: "Many hospitals have cut bed numbers as part of their financial recovery plans and attempts to balance their books.
"This means that there are fewer available beds for patients coming through A&E who need to be transferred within four hours to a hospital ward from the emergency department to meet the Government's access target."
He added: "Working towards the four-hour target on A&E waiting times has been a fantastic achievement, it has proved good news for patients and the extremely long waits seen in the last decade are now very rare.
"However respondents tell us that despite this success, the level of performance in many departments is proving unsustainable and these departments are finding it difficult to cope on a daily basis."
Doctors also raised concerns about the effects of the target on clinical care with two-thirds saying that some patients are moved to inappropriate areas or wards and over half reporting that patients may be discharged before they had been adequately assessed or stabilised.
Previous surveys by the BMA have shown that many believe the target is set too high and that a lower goal of 95% of patients being seen within four hours would be more realistic.
Mr Martin Shalley, President of BAEM said: "Attendances at urban A&E departments continue to rise and pressure on beds remains a significant factor for achieving the four-hour target.
"It is vitally important to separate acute and elective facilities so that each can perform efficiently and improve the patient journey."
A spokesperson for the Department of Health said fewer beds were needed because patients were being treated more efficiently.
"But where the NHS needs more beds, there are more beds. Also, with fewer patients being kept in overnight, the number of day-only beds has increased by over 40% since 1997."
They added: "Only four years ago, 25% of patients spent more than four hours in A&E. Now, 98.5% of patients are seen, diagnosed and treated within four hours.
"This is a fantastic achievement that has transformed patient care, and is testimony to the hard work and professionalism of thousands of frontline NHS staff."
The survey was sent to doctors throughout the UK but the majority of responses were from those working in England.
Targets were found to vary in other areas of the UK.