As news of the wedding spread, so did questions about how the prime minister, who is now on his third marriage, managed to wed in a Catholic church.
Ms Symonds is Catholic and has previously spoken about her faith on social media.
An expert has said Mr Johnson may never have previously married in the eyes of the Catholic Church.
Neither his six-year first marriage to Allegra Mostyn-Owen, nor his second 27-year marriage to Marina Wheeler were Catholic ceremonies, and in the eyes of the Church they may be invalid.
Matt Chinery, an ecclesiastical and canon lawyer, told Times Radio on Sunday: “There is a requirement if you are a Roman Catholic that you must have your wedding overseen by a Roman Catholic priest or deacon.”
He explained that any other form of marriage "is not valid unless you had previous permission from your bishop to marry outside of the Catholic church".
Boris Johnson was baptised as a Catholic, but was confirmed as an Anglican as a teenager.
Mr Chinery explained: “What I think has happened here is that the Catholic Church has looked at Boris Johnson's first two marriages [and] seen that he’s a Roman Catholic by baptism.
“They’ve looked at that, said ‘you are a Catholic because you were baptised Catholic, your first two weddings weren’t in a Catholic church overseen by a Catholic minister, therefore the Roman Catholic church does not recognise those two marriages as valid’.”
He added: “So in the eyes of the Catholic Church Boris Johnson woke up last week as somebody who wasn’t married and had never been married and so was free to marry in the cathedral this weekend.”
Catholic writer Catherine Pepinster was of the same view.
She told The Mail on Sunday that as Mr Johnson’s previous marriages were “in non-Catholic settings”, the church did not recognise them.
“As far as the church is concerned, this is his first marriage,” she said, adding: “The others don’t need to be annulled.”
Initially, Downing Street would not comment on reports of the wedding but on Sunday morning a spokesperson said: “The prime minister and Ms Symonds were married yesterday afternoon in a small ceremony at Westminster Cathedral.
“The couple will celebrate their wedding with family and friends next summer.”
They are said to have sent save-the-date cards to family and friends for the celebration on 30 July 2022.
The wedding ceremony at the Catholic cathedral was carried out by Father Daniel Humphreys.
The couple announced their engagement – together with the news that they were expecting their first child – in February last year.
He and Ms Symonds have a son, Wilfred, who was born in April 2020.
Mr Johnson finalised his divorce from his second wife Marina Wheeler in 2020.
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