One in three NHS patients have been forced to chase results for x-rays, MRIs and other tests, new research has revealed.
A joint report by three health policy think tanks has found that a spate of administrative errors in the NHS is putting people off getting care when they need it.
These "frustrating" issues can seriously impact patient safety and will mean the Government fails to meet its backlog pledge, experts warned.
Other NHS admin failures included patients not being kept up to date about waiting times, receiving an appointment letter after the date it was supposed to have taken place, and not knowing who to contact for help while waiting for care.
It comes as Sir Keir Starmer declared victory on his manifesto pledge to deliver an extra two million NHS appointments.
The Prime Minister said his Government was "delivering on our promise to fix the NHS and make sure people get the care they need" with the waiting list down by 160,000 since the General Election in July, to 7.46 million.
"We said we'd turn this around and that's exactly what we're doing - this milestone is a shot in the arm for our plan to get the NHS back on its feet and cut waiting times," he said, adding: "But we're not complacent and we know the job isn't done."
However, a report by The King's Fund, Healthwatch England and National Voices said the "day-to-day dysfunction in how the NHS communicates" with patients is still having a negative impact on people's experience of the health service.
They said communication failures were "driving perceptions of an organisation that is wasting money, time and staff".
The research included a poll of 1,888 adults in England conducted by Ipsos.
Of the 1,622 people who said they had used NHS services in the previous 12 months - either for themselves or someone they care for - 64 per cent reported issues with communication.
A third said they had to chase results following a test, scan or X-ray, while the same proportion were not kept updated on how long they would have to wait for treatment.
Nearly a quarter said they did not know who to contact while waiting for care, while 20 per cent said they received a letter for an appointment after it was supposed to take place.
Julia Cream, a policy fellow at The King's Fund and co-author of the report, said the findings "lay bare the day-to-day dysfunction of an NHS that is too often not meeting people's needs".
"Behind these numbers are stories of people who are worried about their health and struggling to get through the NHS's front door," she said.
"The Government is trying to bring down waiting lists and improve access but these efforts will fail if the NHS cannot communicate effectively with people about when their appointment is or who they need to contact."
The survey also found that of the people who had experienced at least one admin issue, 42 per cent said they would be less likely to seek care in the future.
Almost half said it left them thinking the quality of the care the NHS provides is "poor".
Louise Ansari, chief executive of Healthwatch England, said: "We've heard countless stories of people whose NHS letters arrived after the appointment day or were sent to the wrong address, while some had their referrals lost in the system.
"Administrative errors are frustrating and can have serious implications for patient safety."
Last year, The Telegraph revealed research suggesting that one in four of the eight million missed hospital appointments each year - at a cost of £1 billion - were because letters arrived after the appointment date.
Dr Victoria Tzortziou-Brown, vice-chairwoman of the Royal College of GPs, said admin errors add to the "bureaucratic workload" of family doctors.
"GPs and our team members are often involved in chasing up test results, missing letters, and appointments with our patients, adding to our bureaucratic workload at a time when need for our care is rocketing, and we have a severe shortage of GPs," she said.
Helen Morgan MP, health spokeswoman for the Liberal Democrats, said the "shocking" problems were "not without consequence".
"It puts patients off going for vital check-ups just to avoid going through this torrid ordeal all over again. The Conservative Party's beyond-shameful neglect has pushed our NHS to the brink, but the Labour government's failure to step up and address this crisis properly is inexcusable," she said.
While celebrating its success for delivering an extra two million appointments, scans and tests in just five months, Downing Street also said it would give performance bonuses to the best performing NHS hospital trusts.
According to No 10, there would be an extra £40 million funding pot for trusts who make the biggest improvements in cutting waiting lists, which could then be spent on improving its hospitals' technology and infrastructure.
An NHS spokesman said: "This timely report highlights yet another consequence of the long-term capital starvation faced by the NHS, with too many organisations still struggling with outdated tech and data systems to support effective patient administration.
"We have begun to address this through investment in front-line digitisation and the NHS app but we know we need to go much further to make sure patients have the information they need in the right place and in a timely way."
A Department of Health spokesman said: "We are upgrading the NHS app, so patients can book and rearrange appointments, choose which hospital to be treated at, receive test results, and choose if they want to be seen in person or remotely."