Mrs May, 59, who backed staying in the EU, has been home secretary since 2010. Mrs Leadsom, who campaigned to leave the EU, said the UK needed “strong and stable government” and that Mrs May was “ideally placed” to implement Brexit.
A statement is expected from its chairman, Graham Brady, at about 17:00 BST.
Mrs May is also expected to make a public statement, at about 18:00 BST, according to BBC assistant political editor Norman Smith.
In a speech earlier on Monday setting out her leadership campaign platform, Mrs May – who rejected the argument that the next leader and prime minister had to have been someone on the winning side of the EU referendum – said: “Brexit means Brexit and we’re going to make a success of it.”