LONDON, Sept. 5 -- The brother of British Prime Minister Boris Johnson announced his resignation Thursday as an MP and as business minister.
In what was described as yet another blow for the embattled prime minister, Jo Johnson said he was unable to resolve family loyalty and the national interest.
In a message on his social media site, Jo Johnson said: "In recent weeks I have been torn between family loyalty and the national interest -- and it's an unresolvable tension, and time for others to take on my roles as MP and minister."
He said it had been an honor to represent the parliamentary seat of Orpington and to serve as a minister under three prime ministers.
The Daily Mail in London said his decision to stand down suggested he did not believe his brother was acting in the national interest.
Downing Street was said to have been taken by surprise by Jo Johnson's decision.
A spokesperson said in a brief statement: "The prime minister would like to thank Jo Johnson for his service. He has been a brilliant, talented minister and a fantastic MP."
"The Prime Minister, as both a politician and a brother, understands this will not have been an easy matter for Jo."
Jo Johnson has had a much lower profile in politics than his controversial brother, and he supported Britain remaining in the European Union (EU).
He served as a minister in Theresa May's Conservative government but resigned accusing May of bungling Brexit negotiations with the EU.
He returned to frontline politics after his brother appointed him in July as a business minister.
The Daily Mail said Jo Johnson has not expanded on his reasons for resigning but is thought to have been infuriated by the way the prime minister fired 21 Conservative rebels who voted with the opposition benches this week.