Former British prime minister David Cameron had earlier described it as "deeply shameful" during a visit in 2013. He, notably, also stopped short of an apology.
British Prime Minister Theresa May on Wednesday expressed "deep regret" for a massacre by British troops in India in 1919. She, however, stopped short of an apology.
"We deeply regret what happened and the suffering caused," May told the British parliament, as India prepares to mark the 100th anniversary of the killings.
Jeremy Corbyn, leader of the main opposition Labour Party, called for "a full, clear and unequivocal apology".
The April 13, 1919 Jallianwala Bagh massacre, in which British troops opened fire on thousands of unarmed protestors, remains an enduring scar from British colonial rule in India.
Colonial-era records show about 400 people died in Amritsar when soldiers opened fire on men, women and children in an enclosed area. Indian figures, however, says at least 1,000 people were killed.
Former British prime minister David Cameron had earlier described it as "deeply shameful" during a visit in 2013. He, notably, also stopped short of an apology.
A ceremony is due to take place at the site of the massacre on Saturday.
