The Archbishop of Canterbury has tested positive for Covid-19 and will miss the service of thanksgiving at St Paul’s Cathedral to mark the Queen’s Platinum Jubilee.

The Most Revd Justin Welby was diagnosed with mild pneumonia on Thursday but had continued to work. However, after developing symptoms over the weekend he tested for coronavirus.

The head of the Church of England is resting at home and has cancelled all engagements this week.

Buckingham Palace has been told of his regret that he will not be able to take part in the service, Lambeth Palace said.

The Archbishop of York, the Most Revd Stephen Cottrell, will deliver the sermon at St Paul’s on Friday instead.

Mr Welby said: “I am deeply saddened to be missing the historic celebration of Her Majesty the Queen’s Platinum Jubilee.

“However, I will be praying for the Queen and giving thanks for her extraordinary 70 years of service to us all.

“I will also be praying for our nation at this time of celebration and thanksgiving. May the Queen’s example bring us together in unity and care for one another.”

He added that he hopes the service on Friday will inspire those watching “with the Queen’s profound commitment to fostering unity and peace among all people – and to promoting care for the natural world that God has given us”.

“Led by the love of Jesus Christ, the Queen has lived her life for the benefit of others: l pray we find inspiration from Her Majesty over the Jubilee weekend and long into the future,” he said.

“Meanwhile, as we continue to live with coronavirus, I pray too for all those who are still suffering and everyone who continues to mourn loved ones they have lost during the pandemic. May you each know the love and comfort of God.”

It is unclear if the Queen will attend the thanksgiving service at St Paul’s, but if she does there will be no ceremonial journey to the cathedral and she will use a different entrance from the steep main steps.

Wider members of the royal family are expected to be present including possibly the Duke and Duchess of Sussex and the Duke of York, and the service will be followed by a Guildhall reception for them.

Mr Welby was also supposed to light a Jubilee beacon at Lambeth Palace on Thursday evening, but it will be lit on his behalf, Lambeth Palace said without naming the person who will take his place.