A scientist leading the development of a coronavirus vaccine said it is “not possible to predict” when it will be ready for the wider population.

The Government said that if the Covid-19 vaccine candidate being developed by Oxford University proved successful in human trials, then up to 30 million doses could be available for the UK by September.

But Professor Andrew Pollard, head of the Oxford Vaccine Group, has now said it was “very difficult” to know when scientists will have proof that it is effective.

Work began in January on the vaccine, which uses a virus taken from chimpanzees and has been developed by the University of Oxford’s Jenner Institute and the Oxford Vaccine Group.

The first phase of trialling involved 160 healthy volunteers between 18 and 55.

Now scientists want to recruit up to 10,260 people across the country for phases II and III, which involve vastly increasing the number of volunteers and expanding the age range to include older adults and children.

Appearing on BBC Radio 4’s Today programme on Friday, Prof Pollard was asked what the chances were of having a proven vaccine within a few months.

He said: “It is a very difficult question to know exactly when we will have proof that the vaccine works because we need, within our population of 10,000 people, to have enough of those who have been exposed to the virus over that time, who are hopefully in the controlled group, who are getting the control vaccine, and to see whether the coronavirus vaccine protects them.

“Now, there is uncertainty about how many cases there will be over the next three months.

“If there are cases then it is certainly possible by the autumn to have a result, and that is what we are hoping for, but it is not possible to predict.”

Researchers will assess the immune response to the vaccine in people of different ages, to determine how well the immune system responds in older people or children.

Adult participants in the phase II and phase III groups will be randomised to receive one or two doses of either a vaccine known as ChAdOx1 nCoV-19, or a licensed vaccine (MenACWY) that will be used as a ‘control’ for comparison.

ChAdOx1 nCoV-19 is made from a weakened version of a common cold virus from chimpanzees that has been genetically changed to make it impossible for it to grow in humans.

This has been combined with genes that make proteins from the Covid-19 virus (SARS-CoV-2) which play a key role in the infection pathway of the SARS-CoV-2 virus.

Prof Pollard said in a statement that clinical studies were progressing “very well”.

Sarah Gilbert, professor of vaccinology at the Jenner Institute, said: “We have had a lot of interest already from people over the age of 55 years who were not eligible to take part in the phase I study, and we will now be able to include older age groups to continue the vaccine assessment.

“We will also be including more study sites, in different parts of the country.”

Production of the vaccine has already been scaled up ahead of the trial to prepare as early as possible for potential future deployment.

AstraZeneca said this week it had the capacity to manufacture one billion doses of the University of Oxford’s potential Covid-19 vaccine and could begin supply in September.