The government must set out plans for an inquiry into its handling of the coronavirus pandemic, the health service ombudsman has said.
This was not about blaming staff but about "learning lessons", he said.
Ombudsman Rob Behrens said patients were reporting concerns about cancelled cancer treatment and incorrect Covid-19 test results.
Ministers have not committed to holding an inquiry, but have accepted there are lessons to be learned.
The Parliamentary and Health Service Ombudsman (PHSO) stopped investigating complaints against the NHS on 26 March, to allow it to focus on tackling the Covid-19 outbreak.
But people had continued to phone in with these concerns, Mr Behrens said.
And cancelled treatment and wrong coronavirus test results have emerged as major themes.
"Complaining when something has gone wrong should not be about criticising doctors, nurses or other front-line public servants, who have often been under extraordinary pressure dealing with the Covid-19 crisis," he said.
"It is about identifying where things have gone wrong systematically and making sure lessons are learned so mistakes are not repeated."
Mr Behrens said he had written to the government on 19 May asking for information about the scope of any future inquiry, but had not received a response.
Hearing the real experiences of people who used NHS services during the pandemic should form part of any future review of the government's handling of the pandemic, he added.
And an "independent, swift and urgent" review could have an impact on policies should there be a second wave of infections.
He said while the government still needed to focus on the current crisis, there were already themes "that we can learn from".
"You can do both things," he said.
Last month a group of leading scientists and medical experts wrote to the government, demanding an urgent public inquiry into the response to Covid-19. They warned that without it more lives could be lost if there was a second spike in cases.
Relatives of 450 people who have died in the pandemic have also demanded an immediate public review to minimise the continuing effects of the virus, ahead of a full inquiry.
And a number of MPs have said they will form a cross-party parliamentary group in support of an urgent inquiry into the government's handling of the crisis.
During an evidence session to MPs on Tuesday, patients described problems they had had because of cancelled care.
Knee-surgery patient Rob Martinez said he had not heard anything from his doctors.
"It just went so silent. I was so close to having it and then it got cancelled and it was absolutely devastating," he said.
Daloni Carlisle said: "My doctors told me that I needed some chemotherapy. I then fell into a hole where I was absolutely in limbo.
"I'd had absolutely no communication about when this chemotherapy might start. So for most of the lockdown I've been sitting here at home knowing that all the cancer is growing, knowing that the tumours in my lung, in my liver, in my spine are all busily growing and absolutely no word from the hospital about when some treatment might start.
"I can't tell you how difficult that limbo period has been."
Mr Behrens said people should report their complaints to the PHSO office if they had not been resolved by the local service's own complaints process, "otherwise other people may experience the same failings".
Health Secretary Matt Hancock on Tuesday tweeted that the Nightingale hospitals, set up to care for coronavirus patients if existing hospitals overflowed, would be converted into cancer-testing centres.
Follow Rachel on Twitter