It means Home Secretary Theresa May is the only candidate - potentially clearing the way for her to be named Tory leader and PM as early as today.
However, it was not immediately clear whether Conservative rules would require the party's 1922 Committee to seek another contender to appear on the ballot paper alongside Mrs May for a planned vote of the party's 150,000 members.
News of Ms Leadsom's planned statement came moments after Ms May launched her national campaign with a speech in Birmingham in which she presented herself as the candidate of unity and experience, with the backing of an "overwhelming" majority of Tory MPs at Westminster.
The Home Secretary took top slot in the MPs' ballot last week with 199 votes to her rival's 84.
Ms Leadsom's campaign got off to a disastrous start after she was forced to apologise to Ms May for a newspaper interview in which she appeared to suggest that being a mother gave her an edge over the childless Home Secretary as a future prime minister.
During the campaign she said: "I am a Christian and I am proud of it.
"It absolutely acts in the background in my desire to have a very honest campaign with high integrity and so on."