THERESA May's local paper has published a letter from more than 70,000 people in Ireland and Britain calling on the prime minister to "protect peace in Northern Ireland".
The letter is included in a full-page advertisement in the Maidenhead Advertiser and was paid for by members of Uplift, which styles itself a "people-powered a community of over 206,000 people in Ireland".
The campaign group co-ordinates petition signings and helps people contact their TDs with the aim of building a "more equal, sustainable and just Ireland".
The letter is signed by 72,833 people, who urge Mrs May to ensure Brexit does not put the peace process in jeopardy.
Uplift director Siobhan O’Donoghue said: "The Troubles cost thousands of lives and tore families apart. Now, the fragile peace we all built could be under threat by Theresa May’s shocking Brexit dealings."
Ms O'Donoghue claimed ordinary people's voices had not been heard "when it comes to the risk of losing our precious peace process".
"Everyone knows that achieving peace in Northern Ireland was no easy task, it took people who were at war coming together to work on a shared goal of peace and reconciliation." she said.
"Massive sacrifices were made so that a fragile but hopeful peace could be achieved – we want to make sure Theresa May is reminded of this today."
The initiative was also supported by 38 Degrees, a not-for-profit political-activism organisation based in Britain.
