He was the longest serving minister in the cabinet and served three different Prime Minister and four Conservative governments. F from the Government, ITV Wales takes a look back at Alun Cairns' career.
Alun Cairns began his political career standing as a candidate for parliament in 1997 for the Gower constituency but lost to Labour.
But in 1999 he became an AM in the Welsh Assembly, where he sat for 11 years.
He was the Conservative Party's candidate for the Vale of Glamorgan in 2005 and was re-selected as Parliamentary candidate for the seat in July 2007.
He briefly resigned from his post in Shadow Cabinet in 2008 after a comment made about Italians on a BBC radio show. He was re-appointed four months later to the Shadow Cabinet.
He was elected as the MP for the Vale of Glamorgan in May 2010, gaining the seat from Labour.
He became Secretary of State for Wales in March 2016 serving first under David Cameron.
Under David Cameron, Alun Cairns campaigned to remain in the EU during the 2016 Brexit referendum - but changed his mind under Theresa May and [**Boris Johnson.**](http://Alun Cairns remains as Welsh secretary under Boris Johnson)
Prime Minister Boris Johnson responded to his resignation letter and remarked on Mr Cairns' commitment to Wales. The Prime Minister thanked him for his support:
Mr Johnson gave Alun Cairns credit for his work in "ensuring the abolition of tolls on the Severn bridges".
Mr Cairns was also behind the controversial renaming of the Second Severn Crossing to The Prince of Wales Bridge.
He said the decision was made to "formally recognise his commitment and dedication to Wales and the UK as the Prince of Wales."
In his resignation letter Mr Cairns has said he will "cooperate in full with the investigation" and is "confident" he will be cleared of all wrong doing. He said it was an honour to serve in Mr Johnson's Government.