The family of 19th Century prime minister William Gladstone have travelled to the Caribbean to apologise for the part an ancestor played in the slave trade.

William was the son of John Gladstone, who was one of the largest slave owners in the British West Indies. In a joint statement by descendants, it said that they believed his actions amounted to being a crime against humanity and they hoped to make a better future.

It has been reported that Charlie Gladstone, whose family's ancestral home is Hawarden estate in north Wales, is due to travel to Guyana, South America, with five other family members to make an apology for John's ownership of Africans.

Their journey coincides with the 200th anniversary of the 1823 rebellion in Demerara, a British colony that later became part of Guyana. It started on one of Gladstone's plantations - some historians argue its violent suppression had a role in bringing an end to slavery.

The statement, by two generations of descendants, said John Gladstone "held the people of Guyana in slavery and was highly instrumental in bringing indentured labour to Guyana too".

"We believe that his actions amounted to a crime against humanity and wish to apologise to the people of Guyana. We know that we can't change the past, but we believe that we can make a better future." The Gladstone family plan to make their official apology at the opening of the University of Guyana's International Institute for Migration and Diaspora Studies, which it said it hopes to help fund with a grant of £100,000.

"For us this isn't just about money though. It is about acknowledging that the slavery still has a massive impact on many people's health and wider socio-economic status across the world," the statement said.

Rob Gladstone, Charlie's brother, called on the UK government to begin "reparative justice" by apologising for slavery within the British Empire. The Atlantic slave trade saw millions of Africans enslaved and forced to work, especially on plantations in the Caribbean and Americas, for centuries from about 1500.

The British government and the monarchy were prominent participants in the trade, alongside other European nations. Britain also had a key role in ending the trade through Parliament's passage of a law to abolish slavery in 1833.

Rishi Sunak has refused to apologise for the UK's role in slavery. Asked at Prime Minister's Questions in April by Labour MP Bell Ribeiro-Addy whether he would offer a "full and meaningful apology", Mr Sunak said he would not, and that it was important to "have a society that is inclusive and tolerant from people from all backgrounds".

"Trying to unpick our history is not the right way forward and is not something we will focus our energies on," he said.