Mr Johnson has spoken to Tory 1922 Committee chairman Sir Graham Brady and agreed to stand down, with a new Tory leader set to be in place by the party conference in October, a No 10 source said.
His resignation follows a string of ministerial resignations in protest of his leadership.
On Thursday morning, recently appointed Chancellor Nadhim Zahawi and close ally defence secretary Ben Wallace both called for him to quit.
What time will Boris Johnson make a statement?
The No 10 spokesman said: “The Prime Minister will make a statement to the country today.”
The Prime Minister will publicly announce his resignation, likely before lunchtime, according to reports.
How to watch Boris Johnson resignation statement
Boris Johnson will likely officially announce his resignation outside No 10 where he will announce he will remain as caretaker Prime Minister until the Conservative party conference in October.
The statement will be available to view on the usual channels, all major news channels including BBC and Sky News.