General Election 2019: Welsh Secretary resigns from cabinet after former aide linked to rape trial collapse
The Cabinet minister's decision to quit will rock the Conservative Party which is officially launching its election campaign on Wednesday.
In a letter to Prime Minister Boris Johnson, Mr Cairns said: "You will be aware of allegations relating to the actions of a Party employee and candidate for the Welsh Assembly elections in the Vale of Glamorgan.
"This is a very sensitive matter, and in light of continued speculation, I write to tender my resignation as Secretary of State for Wales.
"I will cooperate in full with the investigation under the Ministerial Code which will now take place and I am confident I will be cleared of any breach or wrong doing."
Mr Cairns claims he had been unaware of former staff member Ross England's role in the collapsed trial until after the story broke last week.
BBC Wales said it had obtained a leaked email sent to Mr Cairns which showed he had been made aware of the allegations as early as August last year.