British Prime Minister Boris Johnson thanked his brother, Jo, for his service after the younger sibling quit his ministerial role and said he would stand down from parliament, citing a conflict between family loyalty and the national interest.

“The prime minister would like to thank Jo Johnson for his service,” a spokesman from Johnson's office said in a statement.

“He has been a brilliant, talented minister and a fantastic MP (Member of Parliament). The prime minister, as both a politician and brother, understands this will not have been an easy matter for Jo.”

Johnson's plan to kick off what is in effect an election campaign casting parliament as the enemy of Brexit was overshadowed on Thursday when his younger brother quit the government, citing the national interest.

As the United Kingdom spins towards an election, Brexit remains up in the air more than three years after Britons voted to leave the bloc in a 2016 referendum. Options range from a turbulent 'no-deal' exit to abandoning the whole endeavour.

Ahead of a speech in northern England where the prime minister was expected to begin an informal election campaign, his own brother, Jo, resigned as a junior business minister and said he was stepping down as a lawmaker for their Conservative Party.

“In recent weeks I've been torn between family loyalty and the national interest — its an unresolvable tension and time for others to take on my roles,” he tweeted.

The 47-year-old, who campaigned to remain a member of the EU in the 2016 referendum while his older brother was the face of Vote Leave, has been in parliament since 2010, serving in several ministerial roles.

After wresting control of the lower house of parliament on Wednesday, an alliance of opposition parties and rebels expelled from Johnson's Conservative party voted to force him to seek a three-month delay to Brexit rather than leaving without a deal on Oct 31, the date now set in law.