Government figures show that 1,631 people died within 28 days of a positive COVID-19 test on 26 January - the second highest figure recorded in a 24-hour period since the pandemic began early last year.
The total number of deaths within 28 days of a positive COVID-19 test now stands at 100,162 UK-wide, with 88,042 in England, 1,763 in Northern Ireland, 5,796 in Scotland and 4,561 in Wales.
Those who have lost their lives to COVID-19 since the pandemic began include at least 16 GPs, and hundreds of health and care staff overall.
The UK is the first European nation to reach 100,000 deaths and the grim landmark comes 10 months after chief scientific adviser Sir Patrick Vallance said a toll just a fifth of the mark passed today would be a 'good outcome' for the UK.
He told MPs in March 2020: 'If we can get this down to numbers 20,000 and below that's a good outcome in terms of where we would hope to get to with this outbreak.'
BMA chair Dr Chaand Nagpaul said: 'We must not and will not forget this day; the day when we mourn the deaths of 100,000 people from a virus which has wreaked havoc on our health service, nation and society.
'When we are confronted with statistics like this, it’s difficult to fully contemplate the scale of human suffering lying behind them. Behind each statistic is someone who has lost their life: someone’s parent, partner, child, or friend, and the grieving families that they leave behind.
'This has been more painful as the necessity of lockdown and social distancing has constrained families from holding normal funeral services, and from seeking comfort in the company of others. The ongoing impact behind these dreadful figures cannot be underestimated.'
To mark the 100,000 deaths, the BMA has projected a message onto the walls of BMA House, pledging: 'We will never forget'.
Dr Nagpaul said some health and care workers who lost their lives 'were not properly protected from the virus and yet felt under pressure to continue to work and do their duty' in providing patient care.
He added: 'As the first nation in Europe to have reached this dark death toll, we must learn the lessons of this tragedy. We must understand why so many excess lives have been lost in our nation so that we can prevent this scale of death from coming to pass in any future pandemic.
'Now is the time to express our immense sadness at these deaths, to remember the lives they led and the good they did, and to redouble our commitment to reducing the spread of infection, vaccinating the population and protecting one another from this deadly disease.'
Speaking from Downing Street, prime minister Boris Johnson said it was 'hard to compute the sorrow' contained in the grim statistic of 100,000 deaths.
He said: 'I offer my deepest condolences to everyone who has lost a loved one. To all those who grieve we make this pledge: When we have come through this crisis we will come together as a nation to honour everyone we lost and to honour the selfless heroism of all those on the frontline who gave their lives to save others.'
Mr Johnson called the pandemic the UK's 'biggest crisis since the second world war', and pointed to the latest figures on vaccinations showing that 6.8m people have now received at least one dose of COVID-19 vaccine.
He promised that lessons would be learned once the vaccine rollout had 'put us on the course to recovery'.