Boris Johnson reportedly received no warning ahead of brother's resignation

British prime minister Boris Johnson's brother is set to resign as a Conservative party MP over the deepening Brexit crisis.

Jo Johnson, who previously served as minister of state for transport until he resigned last year, confirmed this morning he will step down as an MP as he can no longer support his brother's tenure.

In a short statement on Twitter just after 11.30am, Jo Johnson said it has been an "honour" to serve as an MP and a minister.

However, he said the deepening Brexit crisis has left him "torn between family loyalty and the national interest", leading to an "unresolvable tension" which has led to his resignation.

"It's been an honour to represent Orpington for nine years and to serve as a minister under three prime ministers.

"In recent weeks I’ve been torn between family loyalty and the national interest - it’s an unresolvable tension and time for others to take on my roles as MP and minister," he wrote.

It’s been an honour to represent Orpington for 9 years & to serve as a minister under three PMs. In recent weeks I’ve been torn between family loyalty and the national interest - it’s an unresolvable tension & time for others to take on my roles as MP & Minister. #overandout

— Jo Johnson (@JoJohnsonUK) September 5, 2019

In an apparent reference to the repeated World War II imagery used by Brexiteers, Jo Johnson - who is on the liberal side of the Conservative party - added the hashtag #overandout.

Initial reports in Britain said prime minister Boris Johnson and his office were given no warning of Jo Johnson's decision to announce he will step down as an MP.

The two Johnsons previously clashed last year when Jo Johnson resigned as minister of state for transport, saying he was disillusioned with the government's Brexit strategy and wanted a new Brexit vote with an option to remain in the EU.

At the time, Jo Johnson warned Britain was "on the brink of the greatest crisis" since World War II, meaning his resignation was not entirely unexpected.

However, the resignation of his brother, and the failure to be informed of it before it was announced, is still certain to be seen as another blow to Boris Johnson's time as prime minister.

Last night Boris Johnson was effectively trapped by the House of Commons after MPs voted to block a no deal Brexit, seek legislation to request another Brexit deadline extension from the EU, and to prevent the prime minister from calling an election until the extension is agreed.

The opposition and Conservative party rebel vote strategy took place to stop Boris Johnson from initially agreeing to a pre-Brexit deadline election date before switching it until after the current October 31 deadline in a bid to force through a no deal situation.