Boris Johnson has said he is backing David Cameron in response to former media adviser Andy Coulson's claim he wanted to be Prime Minister.
Former News of the World editor Andy Coulson is being detained at a Scottish police station by officers investigating alleged perjury.
Profile of Andy Coulson, David Cameron's former Director of Communications and ex-Editor of News of the World.
Andy Coulson, Downing Street's former spin doctor, has compared the relationship between Ed Miliband and shadow chancellor Ed Balls to the "shamefully dysfunctional" Tony Blair and Gordon Brown partnership when Labour were in power.
He wrote in GQ: "The prime minister should pray Ed Balls remains shadow chancellor until the election. Appointing him as George's (Osborne) opposite number was the Miliband gift that will keep on giving...
"The Tories must look for the divisions and make the most of them a) because they are most certainly real - always a plus - and b) because it's history repeating itself. We are in this hole at least in part because of the shamefully dysfunctional Blair/Brown relationship.
"Labour's Two Eds dislike each other and each thinks he is smarter than the other.
"The Conservatives should imagine in some detail how it would work if they actually won...and share that vision with the British public."
On Mr Miliband, he wrote: "The prime minister must push him to take positions: expose his strategy (to keep his head down, silently hope that the economy continues to go wonky and, well, just be the other guy), challenge him to take a view on the tricky issues opposition politicians love to duck.
"I'm struck by how detached the opposition front bench appears to be from their leader... I just don't think they rate him very much.
"And if they don't, there's a good chance the public will feel the same way once they get to know him properly."
London Mayor Boris Johnson "desperately wants to be prime minister", according to Downing Street's former spin doctor Andy Coulson.
Mr Coulson went on: "If proof were needed that our PM is a man untroubled by self doubt, it came in his next sentence - 'so I think he'll be a bloody brilliant candidate for us'.
"Stabbing David, or anyone else for that matter, in the back would be distinctly off brand - just not very Boris. He would much prefer to see David fail miserably in the election and ride in on his bike to save party and country."
London Mayor Boris Johnson is desperate to be prime minister but would rather see David Cameron "fail miserably" at the next election than betray him, former government spin doctor Andy Coulson has said.
Mr Coulson, former No10 director of communications, advised Downing Street to support the mayor's good ideas, "advise privately on the bad ones, but only engage publicly if absolutely necessary - and celebrate Boris's considerable successes".
Former News International chief executive Rebekah Brooks, 44, from Churchill, Oxfordshire, also appeared at the Old Bailey charged with conspiracy to commit misconduct in a public office.
The matter was adjourned until next month.
Sun chief reporter John Kay, 69, from north-west London, entered a not guilty plea to conspiracy to commit misconduct in public office.
The cases have been brought as part of Operation Elveden, Scotland Yard's investigation into alleged inappropriate payments and information leaks.
In a separate case, a former police officer pleaded guilty to misconduct in public office.
And in another matter, a public official admitted misconduct in public office.
Former government spin doctor Andy Coulson appeared in court to face charges over an alleged conspiracy to bribe public officials for information.
Coulson, 45, is accused with ex-News of the World (NOTW) royal correspondent Clive Goodman of requesting and authorising payments to public officials in exchange for information, including a royal phone directory known as the "Green Book".
Former NOTW editor Coulson, from Kent, and Goodman, 55, from Surrey, face two counts of conspiracy to commit misconduct in a public office - one between August 31 2002 and January 31 2003, and the other between January 31 and June 3 2005.
Wearing a grey suit with a white shirt and black tie, Coulson appeared for a short hearing along with Goodman and the case was adjourned until next month.
Former Number 10 director of communications Andy Coulson has arrived at the Old Bailey in London.
Coulson is due to appear alongside former News International chief executive Rebekah Brooks, former Sun reporter John Kay and former Ministry of Defence employee Bettina Jordan-Barber.
All four were arrested as part of the Metropolitan Police's Operation Elveden, an investigation into alleged corrupt payments to public officials. They are due to appear for a plea and case management hearing.
Former News of the World employees are looking into whether News International could be forced to pay their legal fees after the newspaper's former editor Andy Coulson won an appeal on the issue, the Guardian reports.
The case may become a precedent for other employees who are also facing charges.
The newspaper's former deputy editor Neil Wallis reportedly said he had told his lawyers to "urgently" look into the possibility of launching a challenge himself.
The former News of the World editor Andy Coulson has arrived at Westminster Magistrates' Court.
Former News of the World editor Andy Coulson has won his appeal against a High Court ruling that News Group Newspapers (NGN) does not have to pay his legal costs arising from the phone-hacking affair.