LONDON (Reuters) - Britain will have to slow its COVID-19 vaccine rollout next month due to a supply crunch caused by a delay in a shipment of millions of AstraZeneca shots from India and the need to test the stability of an additional 1.7 million doses. Supply constraints are the biggest threat to Britain’s vaccine rollout - currently the swiftest among the world’s major economies - and health officials warned that the programme would face a significant reduction in supplies from March 29. “It is true that in the short term we’re receiving fewer vaccines than we had planned for a week ago,” Prime Minister Boris Johnson told a news conference, saying this was because of a delay in a shipment from India’s Serum Institute and because a batch in the UK needed to be retested. “As a result, we will receive slightly fewer vaccines in April than in March, but that is still more than we received in February, and the supply we do have will still enable us to hit the targets we have set,” he said. Earlier, health minister Matt Hancock had said that while Britain was currently in the middle of some “bumper weeks of supply”, a batch of 1.7 million vaccine doses had been delayed as it had to be retested for stability. He didn’t specify the manufacturer. Britain is using vaccines made by Pfizer and AstraZeneca, with 10 million doses of the 100 million ordered from AstraZeneca coming from the Serum Institute. A spokesman for the Serum Institute said it had delivered 5 million doses to Britain a few weeks ago, adding it would “try to supply more later, based on the current situation and requirement for the government immunisation programme in India”. Serum Institute Chief Executive Adar Poonawalla was quoted by the Daily Telegraph newspaper as saying that supplies were dependent on how many doses the Indian government allowed to go to the United Kingdom. But, with Britain already at loggerheads with the European Union over vaccine exports, Johnson struck a conciliatory tone, saying he did not think India had blocked any deliveries and wanted to work with Europe too. A health worker prepares an injection with a dose of Astra Zeneca coronavirus vaccine, at a vaccination centre in Westfield Stratford City shopping centre, amid the outbreak of coronavirus disease (COVID-19), in London, Britain, February 18, 2021. REUTERS/Henry Nicholls/File Photo Pressed on whether the Indian government had stopped exports of vaccine to Britain, Johnson said: “No, no, there is a delay as there often is, caused for various technical reasons, but we hope to continue to work very closely with the Serum Institute, and indeed with partners around the world including on the European continent.” Israel is the leader in vaccinating its population, followed by the United Arab Emirates, Chile and then the United Kingdom - and investors are watching closely to see which economies could recover first. More than half of all adults in England have had their first COVID-19 vaccine. For the United Kingdom as a whole, just under half of adults have had their first dose. VACCINE ROW While Britain tries to secure more vaccines, it is also facing growing anger from the European Union, which on Wednesday threatened to slap a ban on vaccine exports to Britain. Hancock said that European Commission head Ursula von der Leyen should respect contract law and that Britain expected to get the deliveries it had ordered. “There are very significant consequences to breaking contract law,” Hancock said. Britain imports Pfizer’s vaccine from Europe, but despite the spat, Johnson said people should not be anxious about supplies from the EU. “These vaccines are a multinational effort and they are produced as the result of international cooperation and we in the UK will continue to view it in that spirit,” he said. Pfizer and AstraZeneca said on Wednesday their delivery schedules had not been affected. An AstraZeneca spokesman said on Wednesday that the “UK domestic supply chain is not experiencing any disruption”. Britain’s medicines regulator said there had been five cases of a rare type of blood clot in the brain among 11 million people given AstraZeneca’s vaccine but said that it found the benefits of the shot far outweighed any possible risks. England’s Chief Medical Officer Chris Whitty said anecdotal reports suggested that some people had not turned up to vaccination appointments after the shot was suspended in some European countries, but record numbers were still being vaccinated. Hancock denied rumours that the delays would mean no adults would get a first dose of the vaccine in April, but said it was important to make sure there was enough vaccine to give people a second dose within 12 weeks of their first. He also said that Britain was on target to offer everyone over 50 a first shot by mid-April, and a shot to all adults by the end of July. He added that a roadmap for lifting lockdown restrictions in England was unaffected. Earlier, housing minister Robert Jenrick said that supplies would pick up again in May, and Moderna Inc has said it is expecting first deliveries of its vaccine to Britain to start in April. Hancock said Britain expected doses of Moderna’s vaccine to arrive “in the coming weeks”.