Britain's Prime Minister Theresa May leaves 10 Downing Street in London, Britain, July 4, 2018. REUTERS/Simon Dawson

LONDON (Reuters) - Britain’s Prime Minister Theresa May was a “little surprised” by Boris Johnson’s resignation as foreign minister, after he had agreed to her new Brexit plan last week, she said in a letter to Johnson on Monday.

Johnson quit on Monday over Prime Minister Theresa May’s plan for Brexit, warning in his resignation letter to May that the United Kingdom would end up like a colony of the European Union after Brexit.

“I am sorry - and a little surprised - to receive (the letter) after the productive discussions we had at Chequers on Friday, and the comprehensive and detailed proposal which we agreed as a cabinet,” May said in her reply to Johnson.

“If you are not able to provide the support we need to secure this deal in the interests of the United Kingdom, it is right you should step down.”