A Requiem Mass will be held for Father Dermott Donnelly, the brother of TV star Dec.
The popular priest, who had recently celebrated 30 years of service in the Catholic church, died earlier this month in hospital after falling seriously ill.
After the tragedy, Declan Donnelly said he was “heartbroken”, adding: “We are all beyond devastated.”
His TV sidekick Ant McPartlin added: “Fr Dermott you were the kindest, wisest man I knew. Rest in peace my friend. You will never be forgotten.
“My thoughts, prayers and love are with your wonderful family.
“The world has lost a special man.”
The Requiem Mass was being held at St Mary’s Cathedral, Newcastle, on Friday lunchtime.
His body was received into St Michael’s Church in the city’s West End on Thursday evening.
The Donnellys – Dermott, Dec, Martin, Eamonn, Camelia, Patricia and Moira – grew up in Cruddas Park and attended St Michael’s when they were growing up.
It was the church where Dec married Ali Astall in 2015, officiated by his brother Dermott.
Such were the numbers of mourners expected at the cathedral that the Mass was being live-streamed to St Michael’s.
The priest, who was 55, was best known for his work with young people and youth ministry. He was a member of the clergy for St Joseph’s Church in Stanley, County Durham.
Hexham and Newcastle Bishop Robert Byrne said: “He truly lived his faith, and in doing so gave so much to others.
“It is with great gratitude that we will remember him and all he selflessly gave.”
During his career, Fr Dermott was a supporter of The Catholic Agency for Overseas Development (Cafod) charity.
Its head of education Monica Conmee said he had been a “great friend of Cafod”, adding: “Fr Dermott chose to stand in the light, and through his immense talent and gifts, so generously guided thousands of young people to experience the love and light of God in their own lives, inspiring them to share this with others.”
In 2014 Fr Dermott featured on Songs Of Praise alongside CBBC’s Hacker T Dog to answer questions about the church that CBBC viewers had sent in.
In April he visited Downing Street and joined other religious leaders praying for peace.