Chinese medics have been filmed undercover for the first time admitting they realised coronavirus could be transmitted between humans but were told by the government to keep quiet.
Senior medics in Wuhan who witnessed some of the first cases of the virus were secretly filmed by a citizen journalist for an ITV documentary.
The medics said they were in no doubt how dangerous the virus was and that they were aware of deaths as early as the end of December 2019.
One said: "Actually, at the end of December or beginning of January, the relative of someone I know died of this virus.
"Many of those living with him were also infected, including people I know."
Another said: "We all felt there shouldn’t be any doubt about human to human transmission."
Speaking after Wuhan was released from lockdown, the professionals also outlined a cover-up by the Chinese government authorities from the start of the outbreak.
One added: "We knew this virus transmitted from human to human. But when we attended a hospital meeting, we were told not to speak out.
"The provincial leaders told the hospitals not to tell the truth."
They said authorities knew that new year celebrations in January would "accelerate the spread of the virus".
"People suggested at city level that it shouldn’t go ahead, but it did because such an event would present a harmonious and prosperous society," the medic said.
The documentary uncovers evidence that while the highly contagious virus was spreading between January 5 and 17 January, no new cases were officially reported in China during that 12-day period.
The Wuhan medical professionals' testimony is backed by leading virologists, including infectious diseases specialist Dr Yi-Chun Lo, the Deputy Director-General of Centers for Disease Control in Taiwan.
He said: "The very early outbreak management was just a mess, a failure.
"I think the pandemic could have been avoided at the beginning if China was transparent about the outbreak and was quick to provide necessary information to the world."
Yet the Chinese authorities were still denying the virus could be passed from human to human.
The programme also speaks to Dr Yin-Ching Chuang, from the Infectious Diseases Prevention and Treatment Network in Taiwan, who said he and his team had been pushing to get to Wuhan.
Two weeks after the official outbreak, Dr Chuang and a colleague got permission to travel.
They were desperate for an answer to one key question - whether there was human-to-human transmission.
But once there, they say they found it difficult to get clear answers, until in one meeting he finally learned the truth.
Dr Chuang said: "While we were at the meeting, we asked a lot of questions, very unwillingly they finally came out and said, 'Limited human-to-human transmission can’t be ruled out.'"
But two weeks after the outbreak officially began, the possibility of human-to-human transmission was still not being publicly admitted.
Dr Chuang said: "What was the scale of infection? How big was this epidemic? How many patients were affected? We didn’t know. Only they knew this.
"Why didn't China inform other countries of this human-to-human matter earlier?"
The Chinese government declined to comment, but have previously said they provided timely information once facts were known.
The documentary also tracks the global story of the first year of Covid-19, tracing the devastation caused by the spread of the virus across four continents.
The speed of the spread of the virus from China to the outside world was described by virologist Sir Paul Nurse, director of the Francis Crick Institute, as like a "forest fire".
He said: "What might’ve taken two, three, four, five years to spread a couple of hundred years ago can spread in 24 hours, it really is like a forest fire in the Australian Outback.
"A potent viral infection is exactly like that."
The documentary also charts how problems with contact tracing in the UK and the lifting of restrictions caused infections to soar.
On March 12 last year, a day after the global pandemic was declared, the UK officially stopped trying to track the outbreak across the country to focus its 1,500 daily available tests on hospitals.
Director of virology Deenan Pillay said: "Community testing of individuals which hadn't been very successful and the contact tracing was stopped, because we didn’t have the capacity to do that."
He said that by lifting lockdown restrictions, in a bid to avert a potential economic disaster, the UK government may have inadvertently helped the virus spread.
He added: "We could rephrase [it], Eat Out To Help Out The Virus, actually.
"That Eat Out To Help Out campaign led to further spread of the virus."
He continued: "At the time of the end of lockdown in July 2020, we needed an environment and an infrastructure that would keep infections down. And the test and trace system was completely failing."
The UK government says it has taken advice from scientific and medical experts throughout.
Dr Pillay issued a stark warning for the future, adding: "The way in which this coronavirus became Covid, transmissions from animals to humans, is going on all the time.
"Live animal trading around the world that fosters this is continuing. And therefore we have got to expect that this will happen again."
Outbreak: The Virus That Shook The World, is on ITV at 9pm on Tuesday, January 19.