The Archbishop of York this evening called for a boycott of all chocolate that is not certified by Fairtrade.

Dr John Sentamu, whose archiepiscopal seat is in the same city as confectioner Nestle Rowntree, said that buying only Fairtrade chocolate would help end the "21st century iniquity" of child labour across the world.

The Archbishop, who has recently returned from a visit to Jamaica, one of the countries worst hit by the slave trade of more than two centuries ago, was delivering a public lecture to hundreds of church and other community members in Hull, constituency of abolitionist William Wilberforce.

Citing research by the Joseph Rowntree Foundation, he said that although child labour is prohibited in the UK, there is a connection with the UK, chocolate manufacturing and child labour because of the conditions under which cocoa is produced. According to the Stop the Traffik campaign for an end to modern slavery, more than 12,000 trafficked children are working on Ivory Coast plantations to produce 43 per cent of the world’s cocoa beans.

Dr Sentamu’s intervention comes just days after high street clothing chain Gap pledged to stop selling thousands of items where child labour may have been used to make them.

The Archbishop launched a "chocolate challenge" where he asked chocolate lovers "to buy only Fairtrade chocolate from now on." He said: "If you can’t find it in your favourite shop, ask to see the manager and say that you will only buy goods which are not produced by slave labour. Which ones? Say you want to trade fairly. Will you promise? If you keep that promise, you could be playing your part in ending a 21st Century iniquity."

Dr Sentamu has deliberately homed in on chocolate, the manufacture of which has a long history in York, because of the British people's known addiction to the substance. UK consumers get through about £4 billion worth of chocolate a year, according to industry statistics.

He did not name individual manufacturers, but his chocolate boycott is in line with reports by the Stop the Traffik campaign, which claims that the only chocolate guaranteed to be produced without child labour is Fairtrade-certified chocolate.

A spokesman for his office said he had deliberately not used the word "boycott" himself because he wanted individuals to make a "positive choice". But he did want them to choose to buy “only” Fairtrade.

The spokesman added: "The suggestion that we should look for Fairtrade goods is positive. He is not denouncing anybody in particular. It should encourage consumers, retailers and manufacturers to redouble their efforts to see that all workers are treated with respect.

“Most people are chocaholics. The Archbishop is asking them to do something, not about their addiction, but their preferences."

The campaign is the latest blow to York chocolate makers Nestle Rowntree, already under attack this month for plans to move Smartie production to Germany with the loss of more than 600 jobs.

York's other chocolate factory, Terry's, was last year shut down by US giant Kraft. A Nestle Rowntree spokesman declined to comment on the use of child labour in cocoa production but said the company's planned changes to the York factory, which includes £20 million worth of new investment, will safeguard the long-term employment for more than 1,800 employees

The irony of Dr Sentamu's assault on Britain's chocolate industry is that most of the leading chocolatiers, such as the Fry, Rowntree and Cadbury families, were Quakers, and many were at the fore of the campaigns to end slavery.

Joanna Scott, of the Biscuit, Cake, Chocolate and Confectionary Association, which represents most leading manufacturers, said: "The cocoa chocolate industry is working in partnership with the governments of Ghana and Ivory Coast and non-governmental organisations on the ground to ensure that the lives of cocoa farmers, children and families are improved and supported in a variety of ways."