David Cameron will on Wednesday become the first serving prime minister to voice regret about one of the British Empire's bloodiest episodes in India and will lay a wreath at Amritsar, scene of a notorious massacre of unarmed civilians.

The 1919 slaughter, known in India as the Jallianwala Bagh massacre, was described by Mahatma Gandhi, the father of the Indian independence movement, as having shaken the foundations of the British Empire. A group of soldiers opened fire on an unarmed crowd without warning in the northern Indian city after a period of unrest, killing hundreds in cold blood.

Cameron's visit and expression of regret for what happened will stop short of an apology - but will make it clear he considers the episode a stain on Britain's history that should be acknowledged.The gesture, coming on the third and final day of a visit to India aimed at drumming up trade and investment, is likely to be seen as an attempt to improve relations with Britain's former colonial possession and to court around 1.5 million British voters of Indian origin ahead of a 2015 election.

Before his visit, Cameron said there were ties of history between the two countries, "both the good and the bad"."In Amritsar, I want to take the opportunity to pay my respects at Jallianwala Bagh," he said, referring to the site of the massacre. Cameron is expected to visit Amritsar's Golden Temple, a place of pilgrimage for Sikhs, and to inscribe his thoughts about the killings in the visitor book.When asked to comment on Britain's colonial past, he said: "I would argue it's a strength, not a weakness. Of course there are sensitive issues, sensitive events, but actually the fact that Britain and India have this history, have a shared culture and a shared language, I think, is a positive."

The British report into the Amritsar massacre at the time said 379 people had been killed and 1,200 wounded. But a separate inquiry commissioned by the Indian pro-independence movement said around 1,000 people had been killed.Brigadier-General Reginald Dyer, the man who gave the order to fire, explained his decision by saying he felt it was necessary to "teach a moral lesson to the Punjab".Some in Britain hailed him "as the man who saved India", but others condemned him. India became independent in 1947.

Many historians consider the massacre a turning point that undermined British rule of India. It was, they say, one of the moments that caused Gandhi and the pro-independence Indian National Congress movement to lose trust in the British, inspiring them to embark on a path of civil disobedience.

"MONSTROUS EVENT"

Other British politicians and dignitaries - though no serving prime minister - have expressed regret about the incident before.In 1920, Winston Churchill, then the Secretary of State for War, called the Amritsar massacre "a monstrous event", saying it was "not the British way of doing business".On a visit to Amritsar in 1997, Queen Elizabeth called it a distressing episode, but said history could not be rewritten. However, her husband, Prince Philip, courted controversy during the visit when he questioned the higher Indian death toll.

Before he became prime minister, Tony Blair also visited, saying the memorial at Amritsar was a reminder of "the worst aspects of colonialism".In recent years, British leaders have begun to apologise for some of the excesses of Empire.Visiting Pakistan in 2011, Cameron angered traditionalists at home saying Britain had caused many of the world's problems, including the Kashmir conflict between India and Pakistan.

When in office, Blair apologised for the 19th century Irish potato famine and for Britain's involvement in the slave trade, while Gordon Brown, his successor, apologised for the fact that British children were shipped to Australia and other Commonwealth countries between the 1920s and 1960s.Britain ruled or held sway in India via the British East India company from the 17th century until 1947.

India's colonial history remains a sensitive subject for many Indians, particularly nationalists who want Britain to recognise and apologise for its excessesOthers believe bygones should be bygones."What happened in the past happened in the past," Aamir Khan, a prominent Bollywood film star, told reporters after a meeting with Cameron on Tuesday.

I don't think we can hold the present generation of Britishers responsible for what happened ages ago. It is not fair. I don't think that they owe us an apology for what happened a century ago."Cameron has said the two countries enjoy a "special relationship", a term usually reserved for Britain's relations with the United States, but it is a relationship undergoing profound change.For now, Britain's economy is the sixth largest in the world and India's the 10th. But India is forecast to overtake its old colonial master in the decades ahead and London wants to share in that economic success.