The UK government has signed deals for a further 90 million doses of coronavirus vaccine.
The vaccines are being developed by the Belgian pharmaceutical company Janssen and the US biotech company Novavax.
It means the UK has placed orders for six experimental vaccines, taking its potential stockpile to 340 million doses.
In theory, there should be enough for everyone in the UK to get five doses. Most of the vaccines require only two.
With most vaccine trials ending in failure, the government is effectively hedging its bets, hoping that at least one of the vaccines it has purchased proves safe and effective.
The price being paid has not been revealed.
Kate Bingham, chair of the UK government Vaccine Taskforce, told the BBC: "We don't know if any of these vaccine formats that we've acquired will actually work. There are no licensed vaccines for any human coronavirus."
She added it was a "priority" to ensure the UK has "sufficient vaccine" for groups "who are most at risk from coronavirus infection".
Business Secretary Alok Sharma said: "The government's strategy to build a portfolio of promising vaccine candidates will ensure we have the best chance possible of finding one that works.
"Today's agreements will not only benefit people in the UK but will ensure fair and equitable access of a vaccine around the world, potentially protecting hundreds of millions of lives."
The government has now purchased experimental coronavirus vaccines that have been developed using four different scientific approaches:
- Adenovirus vaccines: 100 million doses Oxford University/AstraZeneca; 30 million doses Janssen
- mRNA vaccines: 30 million doses BioNTech/Pfizer
- Inactivated whole virus vaccines: 60 million doses Valneva
- Protein adjuvant vaccines: 60 million doses GSK/Sanofi; 60 million doses Novavax
It takes Britain's potential stockpile to a total of 340 million doses - one of the biggest in the world.
The Oxford and BioNTech/Pfizer vaccines are in advanced, phase three clinical trials, with tens of thousands of volunteers recruited.
It is possible that some indication on how effective they are could come in late autumn, but that is not guaranteed.
The government says if the Janssen and Novavax vaccine trials go well, the first deliveries could take place in mid-2021.
The UK has also agreed to co-fund a clinical study of the Janssen vaccine.
By the end of the year, there could be at least half a dozen different coronavirus vaccines in clinical trials in the UK - and members of the public are being encouraged to register their interest online, because without medical volunteers we will not know if any of the vaccines actually works.