LONDON (Thomson Reuters Foundation) - Pope Francis on Thursday urged Catholics in Britain to "open their eyes" and "hear the cries" of human trafficking victims as the Church tackles modern slavery with advocacy and technology.

The pope said he was praying for the freedom of people trapped in slavery, an estimated 40 million globally, and asked Catholics to comfort those who have "survived such inhumanity".

"The Holy Father appeals to us all: that we may open our eyes and be able to see the misery of those so deprived of their dignity and their freedom, and hear their cry for help," Francis said in a public letter to Britain's Bishop John Sherrington.

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At least 13,000 people in Britain are estimated by the government to be victims of slavery - working in car washes, building sites, nail bars, factories and farms - but police say the true figure is more likely to be in the tens of thousands.

The Church has set its sights on human trafficking in recent years, from an alliance of global police chiefs and bishops working to end the crime to a slavery-busting mobile phone app.

The free app, launched by the Church of England and the Catholic Church this week, allows British drivers to report suspected slave labor at car washes amid rising fears that some of the thousands of sites across the nation are abusing workers.

The pope's letter was sent ahead of the Church's annual Day for Life in England and Wales on June 17, which this year focuses on raising awareness about trafficking and slavery.

Francis has spoken out about slavery in recent months, from the "sacred" lives of trafficking victims to branding forcing women into prostitution a "crime against humanity" and asking for public forgiveness for Catholic men who use prostitutes.