Khan, who arrived in Amritsar on Tuesday, paid a rich tribute to those who died in Jallianwala Bagh incident.
"It was incredibly moving to visit Jallianwala Bagh. Our thoughts are with all those who died.
"It is time for the British government to finally apologise. The tragedy in 1919 on the eve of Vaisakhi is one we must never forget," Khan wrote in the visitors' book at the Jallianwala Bagh here.
The massacre took place on April 13, 1919 when British troops commanded by Brigadier-General Reginald Dyer, opened fire at a crowd of unarmed people, on the harvest festival of Baisakhi.
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The former British Prime Minister David Cameron, during his 2013 visit to Jallianwala Bagh, also condemned the tragedy and described it as a "deeply shameful incident" in British history. However, he stopped short of making any official apology.
Wrapping up his first official tour to India, the London Mayor, earlier today, also paid obeisance at the holy shrine of Golden Temple.
During his visit, he participated in langar at the Golden Temple complex and showed keen interest in knowing how the food is prepared.
Khan was also presented with a Siropa (robe of honour) by officials of the Shiromani Gurdwara Parbandhak Committee (SGPC).
Also, sharing the same sentiments on the visitors' book of the Golden Temple, he wrote: " The warmth, hospitality , spirituality are a lesson to us all. Thank you for providing me with memories that will stay with me forever".
On Tuesday evening, Punjab Chief Minister Amarinder Singh hosted a dinner for the London Mayor in Amritsar.