Boris Johnson has announced his resignation as leader of the Conservative Party and UK Prime Minister.

In a statement in front of the famous black door of No. 10 Downing Street, he said he regretted not being able to stay on.

After days of battling for his job, Johnson was deserted by all but a handful of allies after the latest in a series of scandals broke their willingness to support him.

"Of course, it is painful to not to be able to see through so many ideas and projects," Johnson said.

"But as we have seen at Westminster the herd instinct is powerful and when the herd moves, it moves."

"Today I have appointed a cabinet to serve, as I will, until a new leader is in place," Johnson said as his speech was watched by close allies and his wife Carrie.

"I know that there will be many people who are relieved and perhaps quite a few who will also be disappointed. And I want you to know how sad I am to be giving up the best job in the world. But them's the breaks."

Johnson gave no apology for the events leading to his announcement and said his forced departure was "eccentric".

There were cheers and applause as he began his speech, while boos rang out from some outside the gates of Downing Street.