UK Prime Minister Boris Johnson has married fiancée Carrie Symonds in a wedding carried out in secrecy at Westminster Cathedral in London.

The "small ceremony" took place on Saturday afternoon, a Downing Street spokesman said.

The spokesman added that the couple will celebrate again with family and friends next summer.

The stealth affair, described by PA Media as a "secret wedding," was reportedly held in front of close friends and family, according to several British newspaper accounts.

"Congratulations @BorisJohnson and @carriesymonds on your marriage today," the UK Work and Pensions Secretary Therese Coffey wrote in a Twitter post.

Children's Minister Vicky Ford also tweeted a congratulatory message.

"Many congratulations to @BorisJohnson and @carriesymonds & big love to Wilf. So many weddings have been delayed and disrupted by covid. Life is always better with love," Ford wrote.

On April 29, 2020, Symonds gave birth to the couple's first child, Wilfred. In the same month the Prime Minister was hospitalized with coronavirus.

Later in 2020, she made her first joint television appearance with Johnson to thank NHS workers for treating him in hospital and delivering their baby.

The marriage comes just days after the couple were said to have sent save-the-date cards to family and friends for an event on July 30, 2022.

The two announced they were engaged to be married in February 2020.

Johnson, 56, has four children from his second marriage with Marina Wheeler. The two announced they were splitting after 25 years of marriage in September 2018, shortly after which Johnson's relationship with Symonds was confirmed.

He was also previously married to Allegra Mostyn-Owen for six years.

Johnson could marry in a Roman Catholic church because his first two marriages were not conducted under Catholic ceremonies, Matt Chinery, an ecclesiastical and canon lawyer said on Sunday. Some had asked how a twice-divorcee was allowed to get married in a Catholic Church.

Speaking to Times Radio, Chinery said that if you are a Roman Catholic, there is a requirement that you must have your wedding overseen by a priest or deacon and any wedding which is not conducted that way is "not valid" unless there was "previous permission" from your bishop to marry outside a Catholic Church.

"If you are a Catholic, a baptized Catholic and your first two weddings were not in a Catholic Church overseen by a Catholic minister therefore the Roman Catholic Church does not recognize those two marriages as valid," Chinery explained in reference to Johnson.

Symonds, 33, has enjoyed a successful political career, working as part of the campaign team that helped to get Johnson re-elected as mayor of London in 2012.

At age 29, Symonds became the youngest-ever director of communications at the Conservative Party. Analysts say she was crucial in tidying up Johnson's famously disheveled persona during the leadership campaign.