Boris Johnson's brother quitting the government was a brutal blow for the Prime Minister.
The two siblings have so far been able to keep the politics separate from their family life despite stark differences over Europe.
But in a tempestuous week in Westminster the pair's differences proved too much and Jo Johnson, who has represented Orpington for nine years, stood down as both a minister and an MP.
The 47-year-old is the youngest of the four Johnson siblings and shows the radical split in his family.
Jo Johnson is a pro-EU MP who backed a second referendum while sister Rachel stood for anti-Brexit party Change UK.
But Boris, the eldest of the four, backed Brexit in 2016 after chewing over both options and writing a column backing each.
It is the second time Jo Johnson has quit the government in protest at the hardline Brexit plan in less than a year.
He resigned from Theresa May's regime in November, accusing her of offering MPs a choice between “vassalage and chaos”.
Backing a second referendum, he said: "It would be an absolute travesty if we don’t go back to the people and ask if they want to exit the EU on this extraordinarily hopeless basis.”
Jo was supported by Rachel and brother Leo, who said he had demonstrated "full and accurate facts" in a swipe at Boris.
"Brexit has divided the country. It has divided political parties. And it has divided families too," Jo wrote in his resignation letter.
But he then bizarrely rejoined the government just eight months later when his big brother Boris entered 10 Downing Street.
Jo Johnson took back his old job as Universities and Science Minister, this time attending Cabinet, and kept his opinions quiet.
Like his big brother, Jo Johnson attended Eton College under the watchful eye of hyper-competitive father Stanley Johnson.
Stanley - a former MEP- backed Remain in 2016 but later said "the time has come to bail out".
He surprised the political world in 2017 when he joined the reality TV show I'm a Celebrity and struck up an unlikely friendship with George ‘ Toff ’ Toffolo.
The 79-year-old patriarch encouraged his children to battle each other intellectually.
They spoke Greek at home and all four went to Oxford.
Jo later studied in Brussels before joining Deutsche Bank as an investment banker and the Financial Times as a journalist.
Unlike his brother - who was sacked from the Times for lying - he stayed at his first big paper rising to become Paris correspondent, South Asia bureau chief and an associate editor running the influential Lex column.
He swept into politics as an MP at the 2010 election and swiftly became the head of David Cameron's No10 policy unit.
He rose through the ranks as a Cabinet Office, Universities and Transport Minister before the sudden end to his career today.
Jo, who cuts a more cerebral figure than his ebullient older brother, is married to Guardian journalist Amelia Gentleman who exposed the Home Office's Windrush scandal.
In a devastating tweet Jo gave a hint of the family tensions bubbling under the surface.
But his sister Rachel was more explicit.
Speaking at an event last night Rachel said of the family's arguments about Brexit "it's becoming ISIS-like in its intensity and silliness'.
He said: "I think we have an unspoken rule that you don't talk about Brexit. You cant talk about Brexit. He (Boris) is so out numbered. He is the only person who thinks that Brexit is a good idea.
"But how much he thinks it is a good idea is open to question. We believe (other than Boris) in facts.
"We cant have that conversation as it is fact confronting belief or religion. It as come to almost ISIS level in its intensity and silliness frankly."
