* Health minister cites "lumpy" supply
* Health service letter says supply to be constrained for 4 weeks
* Pfizer, AstraZeneca each say delivery schedules unaffected (Adds AstraZeneca comment, context on other vaccines)
By Alistair Smout and Michael Holden
LONDON, March 17 (Reuters) - Britain will see a significant reduction in the amount of COVID-19 vaccines available at the end of the month due to a cut in manufacturing supply, it said on Wednesday, in a setback to one of the world's fastest inoculation rollouts.
Britain is on track to have given a first COVID-19 shot to half of all adults in the next few days and passed 25 million first shots on Wednesday, making it the most extensive and quickest programme so far in a European country.
Health Secretary Matt Hancock said the country remained on track to have vaccinated priority groups by April 15 and all adults by the end of July.
Pfizer Inc and AstraZeneca Plc, makers of the two vaccines being used in the country, each said their delivery schedules had not been impacted, and Hancock played down the setback.
"Vaccine supply is always lumpy and we regularly send out technical letters to the NHS to explain the ups and downs of the supply over the future weeks," Hancock told reporters.
A letter sent around to the state-run health service said there would be a "significant reduction in weekly supply available from manufacturers" from the week commencing March 29, meaning volumes for first doses will be significantly constrained.
Manufacturers predict this will continue for four weeks due to cuts in national inbound vaccines supply, it continued.
The letter said vaccination centres should guarantee no further appointments for April, and focus on ensuring all those in the most vulnerable categories had received their first shot, and those who have already received a dose got a second.
Sterling fell to its lowest of the day versus the dollar in the hour after the letter was first reported, dipping from $1.3882 to $1.3849 but remaining within the range seen in recent days.
Hancock did not say why there would be a reduction in supplies. Britain has been issuing shots made by AstraZeneca and Pfizer/BioNTech, both of which have struggled to meet European Union contract obligations due to production issues.
A spokesman for AstraZeneca said: "Our UK domestic supply chain is not experiencing any disruption and there is no impact on our delivery schedule."
Pfizer, which is contracted to supply 40 million doses to Britain, said it was on course to meet its delivery targets for the first three months of the year, in line with the agreed monthly schedule.
"We will work closely with the government to ensure this remains the case; our overall projected supply for Q2 remains unchanged and we are on course to continue to deliver a steady supply of vaccines to the UK, April through to June, in line with our contractual commitments," a spokeswoman said.
'A UK-FUNDED, UK DELIVERED VACCINE'
Britain has approved a third vaccine from Moderna Inc , with first deliveries expected in the spring, while its medicine regulator is assessing a one-shot vaccine made by Johnson & Johnson.
The opposition Labour Party was critical of the government's response, saying dismissing concerns about the supply issues was not good enough.
The problem comes as Brussels threatens to ban exports of COVID-19 vaccines to Britain to safeguard scarce doses for its own citizens as it aired frustration over a lack of deliveries moving from AstraZeneca in Britain to Europe.
Hancock said the vaccine had been developed with funding from the British government and Britain had a legal right to the shots.
"We set up the supply chain, not just here in the UK but indeed, we helped set up the supply chain in the EU," he said.
"We legally signed a contract for delivery of the first 100 million doses here for people in the UK, as you would expect, both to ensure that people in the UK can get their jab and also because this is a UK-funded, UK-delivered vaccine."
(Editing by Alexandra Hudson, Philippa Fletcher and Bill Berkrot)
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