Boris Johnson will give a speech this morning at the Conservative Party Conference in Manchester.
Boris and Jo Johnson share the family's trademark ultra competitiveness, but in many ways they are very different people.
Jo Johnson will head up the Policy Unit inside Downing Street in a shake-up to inject fresh Tory thinking into Government.
The Prime Minister has hinted that talks are under way over a return to Parliament for Boris Johnson.
David Cameron said he had had discussions about a possible return with the London Mayor, although he stressed that a decision had not yet been taken.
Asked how likely a return was, Mr Cameron said:
– David Cameron
"That's up to Boris. I've had this conversation with Boris and my message to him is 'You're a brilliant Mayor of London, you've done a great job, you've got a lot more to give to public life, and it would be great to have you back in the House of Commons at some stage, contributing to public life'. But that's up to him, but I'll certainly be giving him a warm welcome."
Asked if Mr Cameron could see his former Eton schoolmate make a comeback as a Member of Parliament at the next election, he replied:
– David Cameron
"Absolutely - but that's a matter for him. I think he needs to think about - it's his plan. All I know that he's a massive asset to the country, a massive asset to the Conservative Party. We could make a very strong team together, we do today."
Also quizzed on whether there was a potential job for Mr Johnson within the party, Mr Cameron said:
– David Cameron
"Obviously, this is all dependent on what Boris wants to do - whether he wants another term, whether he wants to become an MP.I think he's got an enormous amount go give to public life, and I don't think he's given up on that idea either. So I think we're quite well aligned if I can put it that way."
The Conservative group on the London Assembly has released a report suggesting Tube stations or Tube lines should be sponsored. They say the extra income from sponsorship could allow fares to be frozen.
It would mean that stations would have their names changed by adding a brand name. The Conservatives carried out a poll which found that 74% of Londoners believe Transport for London should expand sponsorship if it was used to cut or reduce fares.
Stanley Johnson, the father of London Mayor Boris, told ITV News he was "absolutely thrilled" that his son Jo has been added to David Cameron's team in Downing Street.
He said: "I'm absolutely thrilled at this wonderful appointment David Cameron has made, there is no doubting Jo's intelligence.
"He is delighted and I'm confident the Tories will get their act together in a big way.
"I wish my father had been alive to see this and I'll keep lobbying him to make sure the Tories stay green."
David Cameron has tweeted a picture of his new policy board, which features London Mayor Boris Johnson's younger brother Jo.
The Prime Minister's tweet said: "I've been meeting with my new policy board, made up of MPs. I told them I want more ideas to help hardworking people."
The Daily Telegraph's political correspondent Peter Dominiczak has tweeted London Mayor Boris Johnson's reaction to his brother Jo heading up a Policy Unit in Downing Street.
Boris on his brother's promotion : "Great there is finally a Johnson in Number 10. Jo will be superb. Now for Rachel and Leo!"