The UK will not join the EU Covid-19 vaccine scheme, the UK's ambassador to the EU Sir Tim Barrow has said.
Sir Tim said if the UK joined the scheme it would have no say on decisions including on price or which manufacturers to negotiate with.
The UK would also be unable to "pursue parallel negotiations with potential vaccine suppliers", he said in his letter to the European Commission.
The EU scheme aims to secure supplies of potential coronavirus vaccines.
The European Commission plans to enter into agreements with individual vaccine producers on behalf of the bloc's member states as part of the multi-million pound programme.
In return for the right to buy a specified number of vaccine doses in an agreed timeframe and price, the Commission will finance a part of the vaccine producer's upfront costs.
Reacting to earlier reports the UK will opt out of the initiative, the Wellcome Trust said countries "urgently" needed to work together "if we're to stand any chance of delivering global equitable access to a Covid-19 vaccine".
In his letter, Sir Tim said the UK would not participate because the "UK would be required to stop its negotiations with manufacturers with which the EU launched negotiations".
He said the commission had also confirmed it was "not possible for the UK to have a role in the governance shaping decisions on which manufacturers to negotiate with, or the price, volume and delivery schedule negotiated".
Sir Tim added that there could still be collaboration between the UK and the EU on areas such as "sharing of information on promising vaccine candidates" and "vaccine trials" and "manufacturing investments and capacity building".
Earlier, Alex Harris, head of global policy at the Wellcome Trust, said: "The EU vaccine initiative's cap on how many doses participating countries get is the best way to ensure there is enough vaccine for those in need in the rest of the world.
"Delivering vaccine according to need and not who can pay the highest price, is not just morally right, but also the fastest way to end this pandemic.
"We urge the UK government to follow the EU's lead and only secure vaccine doses for those who need it most (healthcare workers, over 65s and other vulnerable groups)."
He said this approach was "critical" for the first "six to nine months of early vaccine availability" as global manufacturing capacity was "unlikely to match demand".
In the UK, human trials of a vaccine are under way in London and Oxford.
Worldwide, about 200 groups are working on vaccines and 18 are now being tested on people in clinical trials.
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Reacting to reports the UK was not going to join the EU scheme, Lib Dem leadership hopeful Layla Moran tweeted: "Walking away from the EU vaccines scheme is putting ideology ahead of public health."
And Labour MP Bell Ribeiro-Addy tweeted: "By refusing to join the EU's vaccine scheme, the government is yet again putting ideology before saving lives."