03/02/2016 09:21 GMT | Updated 03/02/2016 11:59 GMT

Watch David Cameron Tell Boris Johnson He Will 'Go After' His Sister To Get Mayor's Backing Of EU Deal

David Cameron today joked he would “go after” Boris Johnson’s sister in bid to persuade the London Mayor to back the EU deal.

The Prime Minister made the quip about newspaper columnist Rachel Johnson after Labour’s Alan Johnson – who is not related to the pair – pointed out how many members of the Mayor of London’s family backed the UK staying in the EU.

Boris has been less than welcoming of Mr Cameron’s draft renegotiation package with European Commission President Donald Tusk, and this morning said “there is a lot more to do” to get a good deal for the UK.

However his brother, Jo, who is Universities and Science Minister, and father, Stanley, both believe the UK should stay in the EU.

The London Mayor looked relaxed on the Government backbenches today as the Prime Minister delivered a statement to the Commons on the deal, and initially did not appear to want to speak in the debate.

However that changed after Labour’s Alan Johnson, who is leading his party’s campaign to stay in the EU, got to his feet and asked the Prime Minister: “Will he welcome the launch of Environmentalists for Europe today, co-chaired by Stanley Johnson, the father of the Hon Member for Uxbridge and South Ruislip [Boris Johnson]?

“Will he also welcome the splendid article last week setting out the importance for science and technology of remaining in the European Union, penned by his Science Minister, the brother for the Member for Uxbridge and South Ruislip?

“Will he have a word with his Right Honourable Friend and tell him the importance of family solidarity in joining the swelling ranks of Johnsons for Europe.”

Boris drew laughs by shouting out “Am I my brother’s keeper? Am I my father’s keeper?” as the Prime Minister got to his feet to respond.

Mr Cameron said: “Very good. We can’t have too many Johnson’s agreeing with each other.

“There’s also Rachel Johnson, we will have to go after her.”

Rachel Johnson, sister of Boris

Rachel was at Oxford University at the same time as the Prime Minister, but in September last year revealed she was not aware of him at all during her studies.

Writing in the Daily Mail, she said: "The last thing I thought I was interested in at university was foppish ‘hoorays’ who joined all-male dining clubs and threw pot plants through restaurant windows, trashed each other’s rooms, and debagged random diners, baying like bloodhounds.

"I wanted to hang out with brooding, budding playwrights (like Patrick Marber, who wrote Closer) or artists who wore Levi 501s and black rollnecks, and boys who didn’t speak in posh drawls but rather mockney or even a curious, public-school Jamaican patois."

Boris has repeatedly said he believes Britain could thrive outside of the EU, but has yet to clarify whether he will be campaign for the Leave or Remain sides in the referendum.

Read: Six Things We Learned From David Cameron's European Union Draft Deal

When he eventually did get to his feet in the Commons today, he asked Mr Cameron whether the deal would "restrict the volume of legislation coming from Brussels and change the treaties so as to assert the sovereignty of this House of Commons and these Houses of Parliament."

The Prime Minister replied he would look to "put it beyond doubt that this House of Commons is sovereign...at the same time as concluding the negotiations."

He added: "I would argue that, looking across this deal, one can see that we have welfare powers coming back, we have immigration powers coming back, which I have just spoken about, and we have bail-out powers coming back.

"Of course, on the massive return of power that we achieved in the last Parliament with the justice and home affairs opt-out—the biggest return of power from Brussels to Britain since we joined the EU—we have absolutely nailed that down in these discussions to make sure that they cannot get around it.

"Those were all key objectives. I am not saying that this deal is perfect. I am not saying that the European Union will be perfect after this deal—it certainly won’t be—but will the British position be better and stronger? Yes, it will."