LONDON, Dec 8 (Reuters) - An adviser to British Prime Minister Boris Johnson has quit after a video surfaced of her laughing and joking about a party in Downing Street during a Christmas COVID-19 lockdown last year when such festivities were banned. Allegra Stratton, who was most recently Johnson"s COP26 spokeswoman, was his press secretary at the time the video was recorded in December 2020. read more In the video aired by ITV, Stratton was shown at a rehearsal for a daily briefing laughing and joking about a reported gathering at a time when tens of millions of people across Britain were banned from meeting family and friends to celebrate Christmas, or even from bidding farewell to dying relatives. "My remarks seemed to make light of the rules, rules that people were doing everything to obey. That was never my intention. I will regret those remarks for the rest of my days," a tearful Stratton told reporters outside her home. "I understand the anger and frustration that people feel. To all of you who lost loved ones, who endured intolerable loneliness and who struggled with your businesses - I am truly sorry and this afternoon I am offering my resignation to the prime minister."