The battle over controversial reforms to the benefits system turned personal in the House of Commons as David Cameron attacked Birmingham MP Liam Byrne – for being bald.

Speaking to a packed House of Commons, the Prime Minister described Mr Byrne as “Baldemort”, comparing him to the Harry Potter villain Lord Voldemort while also referring to Mr Byrne’s hairline.

He was focusing on Mr Byrne. Labour’s shadow Work and Pensions Secretary and MP for Birmingham Hodge Hill, because he has led Labour’s response to Government plans for a benefits cap.

The Government wants to make sure no family can receive more than £26,000 in benefits if nobody in the household is working.

Labour also supports a benefit cap, but wants to set different limits in different parts of the country to reflect the cost of housing.

This could actually mean the cap would be lower than £26,000 in the West Midlands, although the figure would be set by an independent commission. Mr Byrne has also accused the Government of failing to protect families with children who are disabled.

The insult was delivered by Mr Cameron during Prime Minister’s Questions, the high-profile weekly question and answer session which is always well-attended by MPs.

He asked: “Where is Baldermort? He’s not at home today.”

In fact, Mr Byrne was in the Chamber but the Prime Minister had failed to spot him. The phrase Baldemort first appeared in the comedy film In The Loop, where it was used by fictional spin doctor Malcolm Tucker.

A newspaper has also used the phrase in an article criticising Mr Byrne. The Commons yesterday overturned a series of changes to the Government’s Welfare Reform Bill which had been introduced by the House of Lords.

This included a Lords proposal to exclude child benefit from the £26,000 cap, which would effectively have increased the cap.

Mr Cameron told the Commons: “The cap is right and the cap is fair.

‘‘It is right to say you shouldn’t get more than £26,00 a year in benefits – that’s £500 a week.”