More than a million people from all over Europe are to deliver a petition to Tony Blair and fellow EU leaders calling for changes to the constitution recognising Europe's Christian heritage.
Refusing to accept a secular "fait accompli" from Brussels, a Christian coalition is demanding that each EU state publish its version of the constitution's preamble, with references to God if desired.
Already armed with 1,149,000 signatures and with thousands more pouring in from Holland since the murder of the film-maker Theo van Gogh, the group claims that most states want some reference to Christianity but were blocked by France.
The move is keenly backed by Pope John Paul II, who has repeatedly condemned the "moral drift" of Brussels. "One does not cut the roots to one's birthright," he told pilgrims this summer.
Euro-MPs voted this week to back the calls for a change in the text. Petitioners, led by Italy's International Mission Centre, will now take their case to EU governments. The current version of the preamble eschews Christianity, talking vaguely of "the cultural, religious and humanist inheritance of Europe".
Valery Giscard d'Estaing, the former French president, deliberately left the issue open when he wrote the document, inviting a petition.
"I have chosen not to insert the reference to the Christian heritage in the constitution,"he said. "Rather I appeal to you to persuade me of its necessity."
A British official said it was too late to change the preamble, although national parliaments could add a "rider" stressing their country's Christian roots.
An EU official said: "These Christians could at least have the good grace to accept that they lost the argument."