Ambassador Sally Axworthy described Cardinal Newman as a "giant" of the 19th century whose poetry, sermons and books went beyond a religious audience and spoke to all people.
The Prince of Wales will lead the UK's representation at the open-air ceremony in Rome on Sunday, where Pope Francis will declare the cardinal a saint in front of tens of thousands of pilgrims in St Peter's Square, the first English saint of the modern age.
Ms Axworthy, who is Britain's top diplomat to the Holy See, the jurisdiction of the Catholic Church in Rome, said: "It's an important moment for the Catholic Church but also Holy See relations.
"It's the first British saint canonised in over 40 years and the first post-Reformation saint, and I think Cardinal Newman is important as he's someone who had a global impact.
"I know he has a strong following in the United States, Latin America and certainly a strong following in the Vatican, lots of people have talked to me about how they've been moved by Newman's writings.
"Obviously he had a huge impact on theology, but he was someone who examined his own faith in enormous detail and that meant also asking some difficult questions about having faith in a world which was becoming increasingly secularised - he thought and wrote about that."
London-born Cardinal Newman, who died in England in 1890 aged 89, had been hailed by former Pope Benedict XVI as a model for ecumenism.
An Anglican priest, he renounced an illustrious academic career at Oxford University to convert to Catholicism in 1845, convinced that the truth he sought could no longer be found in the Church of England.
The cardinal went on to found the Oratory at Birmingham in 1848 and through his writings spoke to many about the issues of faith, education and conscience.
In February, Pope Francis approved the second miracle attributed to the cardinal, nine years after his predecessor, Pope Benedict, had beatified the 19th century figure during a visit to Britain.
In the Catholic Church's saint-making process, one miracle is necessary for beatification, and a second miracle, occurring after the beatification ceremony, must be certified by Vatican experts for sainthood to be conferred.
The first miracle involved the curing of Jack Sullivan, a deacon from Boston, Massachusetts, of a crippling spinal disease after he prayed to the cardinal in 2001.
While in 2013 Melissa Villalobos recovered from a torn placenta, that threatened her unborn child's life and her own, after also appealing to the revered priest for help.
Cardinal Newman will become the first saint since the Forty Martyrs were canonised in 1970, after being executed under Reformation laws in the 16th and 17th centuries.
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