Selby and Ainsty MP Keir Mather sworn in to House of Commons

Labour leader Sir Keir Starmer, left, welcomes newly elected MP for Selby, Keir Mather, to the House of Commons

Labour's Keir Mather has become the youngest MP in the Commons after being sworn in on Monday.

Aged 25, he overturned a 20,137 Conservative majority to win the North Yorkshire seat of Selby and Ainsty for Labour in July's by-election.

Hull-born Mr Mather has been nicknamed the "baby of the House".

After swearing the oath of allegiance to the King, Commons Speaker Sir Lindsay Hoyle quipped there were "too many Keirs".

Labour's Keir Mather was elected as MP for Selby and Ainsty in July

Earlier, shadow education secretary Bridget Phillipson said: "I'm delighted that Keir will be joining us in Parliament, making the case for the people of Selby and Ainsty."

According to his party biography, Mr Mather grew up near Selby.

As a child, he joined the youth parliament and set up a Labour group for young people in Hull, his mother Jill Tambaros said previously.

Mr Mather took his seat on the green benches alongside two other new MPs as Parliament returned after the summer recess.

The others were Liberal Democrat Sarah Dyke and Steve Tuckwell, who held the seat of Uxbridge and South Ruislip for the Conservatives after the resignation of former prime minister Boris Johnson.

Ms Dyke has taken over from former Tory MP David Warburton following his resignation after he admitted cocaine use.

The three arrivals came at the start of a busy period in politics, with party conference season weeks away, as well as the King's Speech and two further electoral battles imminent.