The head of the UK Health Security Agency has called the Omicron variant “probably the most significant threat” since the start of the pandemic as she told MPs to expect “staggering” growth rate over the coming days.

Dr Jenny Harries told the Commons transport select committee on Wednesday that it was still too early to judge the potential risk of the new Covid variant and warned that the virus was “growing faster”.

Her stark warning came after the government’s most senior public health adviser, Dr Susan Hopkins, told MPs on Tuesday that Omicron infections in Britain could reach 1m a day by the end of the month.

“It’s probably the most significant threat we’ve had since the start of the pandemic and I’m sure for example the numbers that we see on data over the next few days will be quite staggering compared to the rate of growth that we’ve seen in cases for previous variants,” Harries said.

“The real potential risk here – and I would underline that because we are still learning a lot about the variant – is in relation to its severity, clinical severity, and therefore whether those cases turn into severe disease, hospitalisations and deaths. We’re still at too early stage for that. In fact the world probably is still at too early stage to be clear.”

Harries said that in most regions of the UK the doubling time of the virus was now “under two days” and that its growth rate was being felt in London and Manchester in particular.

She added: “We’re very sure there are levels growing across most communities in the UK now, although there is quite a lot of regional variation still.”

Harries said demand for lateral flow tests has been “absolutely astounding” and that between 6am and 8am on Wednesday alone, the public ordered 200,000 packs. The NHS, she said, was also experiencing “unprecedented demand” for PCR tests.

But she insisted that there was no shortage of lateral flow tests and said the problem in providing them lay in delivery.

Harries said there was capacity for about 650,000 PCR tests a day but that demand varied depending on location. In Wales she said some test sites were at 7% or 8% capacity, whereas in London they were “quite full”.

“People can still order tests at home, and we have put in mobile testing units to try and boost those supplies so people can get tested. So we do have sufficient tests both for PCR and LFTs. But the very rapid rise is causing some temporary pressure.”

Meanwhile, another leading scientist warned that the NHS could be overwhelmed next month and that “very large” numbers of people could be admitted to hospital.

Graham Medley, a professor at the London School of Hygiene & Tropical Medicine and a member of the Scientific Advisory Group for Emergencies, told BBC Radio 4’s Today programme: “If the numbers of infections increasing continues in the way that it has done, and it spills out into older age groups, then we could see numbers of people being admitted to hospital getting very large and certainly going over the 1,000, maybe up to 2,000 a day, that we’ve managed to keep the Delta variant below.”