Gavin Williamson
Britain's Secretary of State for Defence Gavin Williamson arrives in Downing Street in London,16 January 2018. REUTERS/Hannah McKay

Defence Secretary Gavin Williamson has confessed to having an affair with a former colleague that nearly broke his marriage apart.

Williamson, who was made defence secretary after Sir Michael Fallon resigned, said his wife has forgiven him for the fling. The rising Tory start admitted he had a brief office romance with a former colleague, who was married, while a manager at fireplace firm Elgin & Hall in Yorkshire.

"I had a good relationship with everyone I worked with, but with one person this started to develop into something more," he said. "We had to travel together and spent a lot of time in each other's company, the relationship became flirtatious and a couple of times we shared a kiss."

He called the office romance "a dreadful mistake" and insisted it had not progressed beyond kissing "a couple of times".

"It never went further than that, but this had a profound impact on us both and those close to us. It was a dreadful mistake and stopped as suddenly as it had started," he told the Daily Mail.

"I know that this caused problems for her, as it did for me. And having to explain to Joanne what had happened was one of the most difficult conversations of my life. I'll never truly understand how she found it in her heart to forgive me, but I tell myself every day how lucky I am to be with such an amazing person."

Williamson said he quit his job because he did not want a constant reminder of his infidelity. He is understood to have revealed details of the illicit liaison to party officials when he ran for selection as an MP more than a decade ago, the Daily Mail reported. He also told Downing Street about the affair.

The defence secretary's wife gave birth to the couple's first child in late 2004. It is unclear whether Williamson cheated on his wife while she was pregnant. The couple now have two daughters.

Williamson added: "My family means everything to me and I almost threw it away... This incident nearly destroyed two marriages.

"It will always be part of my past, but has not stopped my wife and I from building a wonderful family together."

He said that nearly losing his family made him realise how much it meant to him.

In 2010, Williamson became MP for South Staffordshire. Six years later, Prime Minister Theresa May appointed him as her Chief Whip for his help defending Tory leadership from Boris Johnson. He became the youngest Defence Secretary after Fallon became wrapped up in the Westminster sexual harassment scandal.

Williamson said he did not believe his past infidelity was a resignation matter and denied there was an element of sexual harassment in the relationship, despite being her senior at the firm.