The announcement comes ahead of the results of the European Parliament elections in which May’s party is predicted to come as low as fifth place.
The European Parliament represents people living in the 28 member countries of the European Union (EU).
It has powers in a range of areas that affect member countries and can approve, change or reject new European laws.
May said she had decided to step down in order to allow a successor to implement Brexit, having failed three times to pass her deal with the EU through parliament.
May’s announcement followed a meeting on Friday morning with the Chair of the 1922 Committee of Conservative MPs, Sir Graham Brady.