The massacre took place at Jallianwala Bagh in Amritsar (undivided Punjab) during the Vaisakhi festival on April 13, 1919, when troops of the British Indian Army under the command of Colonel Reginald Dyer opened fire at a crowd of people holding a pro-independence demonstration, leaving scores dead.
At a Vaisakhi reception at Downing Street in London Wednesday evening, Prime Minister May repeated words from her House of Commons statement last month as she referred to the "shameful scar" on British Indian history.
"We deeply regret what happened and the pain inflicted on so many people," she told a gathering of the Indian diaspora.
She said: "No one who has heard the accounts of what happened that day can fail to be deeply moved. No one can truly imagine what the visitors to those gardens went through that day one hundred years ago."
"It was, as the former prime minister H H Asquith described it at the time,'one of the worst outrages in the whole of our history'."
"I do not understand why the British government has not to this day agreed to say sorry," said Lord Loomba, who has also called for an investigation into whether General Dyer instigated the huge atrocity of his own accord or was following orders from higher authorities during the British Raj.
Most recently, during a Westminster Hall debate in the House of Commons to mark National Sikh History Awareness Month, a number of British Indian MPs had repeated calls for a formal apology and expressed the hope that perhaps the Downing Street reception would be the appropriate moment for it.
However, the only reference to the debate made by May noted: I am delighted that last week a debate in Westminster Hall focused on the contribution of Sikhs to the UK following on from the launch of Sikh History and Awareness month by Seema Malhotra MP in April.
"There were some great contributions made during that debate and it was a timely reminder of the hard work, compassion, and generosity of Sikhs in communities up and down the country and abroad."
May admitted that while she had never attended a Vaisakhi Nagar Kirtan, she had heard that they were "fantastic events" when the Sikh community came together to commemorate the birth of the Khalsa.
She added: "But although I haven't yet been to one of these parades I have been lucky enough to be a frequent guest at gurdwaras in my constituency and across the UK and can not only imagine the warm Punjabi welcome at this time of year but just how good the food must be."
"This of course is a particularly important year for the whole of the Sikh community, 2019 marks the 550th anniversary of the birth of Guru Nanak, the first Sikh guru, in 1469. And I am sure we will see many events to celebrate this later in the year."
The Downing Street celebration involved a shabad kirtan performance by students of the Nishkam School in west London.