The Aberdeen South MP last month gave mugs to journalists which mimicked Labour ads, saying: "Controls on family sizes. What's the point of Labour?"

An accompanying note - on Mr Flynn's House of Commons headed paper - read: "The Labour Party has a new range of mugs in production.

"They're made in China - just like Sir Keir Starmer's latest policy."

The mugs were an attack on the Labour leader's refusal to commit to scrapping the two-child benefit cap if his party wins the next election.

The stunt also compared Labour's stance to the one-child policy of the Chinese government, which ended in 2015.

According to the commissioner's website, Mr Flynn is being probed under paragraph eight of the House of Commons code of conduct.

The paragraph states: "Excepting modest and reasonable personal use, members must ensure that the use of facilities and services provided to them by parliament, including an office, is in support of their parliamentary activities, and is in accordance with all relevant rules."

Read more:
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An SNP source hit out at the investigation.

They said: "This farce shows Westminster at its most absurd.

"The two-child cap has pushed 20,000 Scottish children into poverty but instead of worrying about the Tories and Labour Party both backing the policy, Westminster is attacking the SNP for daring to speak out.

"This shameful policy cost families in Scotland £95.7m last year, with typical households losing £3,235 to £6,470 each, but instead of investigating the damage it has caused and Labour's desire to back attacks on those in poverty, Westminster has launched an inquiry into the use of tiny handful of paper, with an approximate value of 2p.

"It tells you everything you need to know about Westminster's skewed priorities."