LONDON (Reuters) - British police said on Friday they would investigate opposition leader Keir Starmer over a potential breach of COVID-19 lockdown rules in 2021 after receiving significant new information, delivering an embarrassing blow to the Labour Party.

       Starmer, who previously served as the country's top prosecutor, has led calls for Prime Minister Boris Johnson and finance minister Rishi Sunak to resign after they both received fines relating to a birthday party celebration thrown for Johnson by staff in his Downing Street office in June 2020.

       An internal report found that staff in Downing Street held a string of boozy parties during COVID lockdowns, angering the public and damaging trust in the country's political system.

       Johnson could receive more fines for other events still being investigated by London police.

       Starmer has also faced increased scrutiny over footage from April 2021 showing him drinking a bottle of beer with colleagues indoors during a visit to the northeast of England.

       The local Durham police force previously said it had concluded that no offence had been established but on Friday it said that "following the receipt of significant new information over recent days" an investigation had been opened.

       A spokesperson for the Labour Party said they were "obviously happy to answer any questions".

       "We remain clear that no rules were broken," the spokesperson said.

       (Reporting by William James and Muvija M, editing by William Schomberg)