LONDON, Dec 8 (Reuters) - Fans in England will need to show proof of double vaccination against COVID-19 or a negative test to attend top-level sport after British Prime Minister Boris Johnson imposed tougher COVID-19 restrictions in the country on Wednesday. The British government has made the NHS COVID Pass mandatory for any event with more than 10,000 people in a bid to slow the spread of the Omicron coronavirus variant. The COVID pass will also be needed for any unseated indoor venue with more than 500 people and any unseated outdoor venue with more than 4,000 people. The rules, announced as part of the COVID "plan B" measures in England, will come into effect from Dec. 15. Johnson said the pass would still be available to people who had received two doses of a vaccine but the British government would keep this under review as the booster programme was extended. Britain has recorded more than 145,000 COVID-19 deaths during the pandemic. A fast start to the vaccine programme helped to contain the impact, and in recent months cases have held steady at around 43,000 daily infections and 135 deaths after Johnson lifted most restrictions in England. But the emergence of the Omicron variant has put governments on edge around the world. Britain has discovered 568 confirmed cases of Omicron. Scotland, Wales and Northern Ireland set their own COVID restrictions and had already imposed tougher rules.