Image copyright Getty Images Image caption Boris Johnson and George Osborne visit the Battle of Britain command bunker

Preparations are reaching their final stage for this year's Conservative conference - and some of it might be surprisingly expensive.

Back in 2011, Boris Johnson told allies that George Osborne had approved an additional £93m for the Metropolitan Police on one condition - the mayor of London must not cause trouble at Conservative Party conference by writing his Monday Daily Telegraph column on a tricky subject that week.

The existence of this deal was first revealed in the Daily Mail serialisation of the biography of the prime minister written by Lord Ashcroft and Isabel Oakeshott.

Most attention generated by that book related to the reliability of allegations about the prime minister's student days. That left surprisingly little attention for this expensive episode, elements of which have been corroborated by BBC Newsnight.

'Best-paid col ever'

A copy of an email seen by Newsnight reveals that the mayor of London specifically committed to avoid writing about Europe in his Daily Telegraph column during conference week.

In the email sent on 29 September 2011, the mayor wrote: "Did deal with George. He gives us £93m for cops. I don't write about euro in Monday Tel and muck up his speech. It's the best-paid col ever."

City Hall had been seeking £93m from the Treasury because it would allow the city to employ more police officers at the end of his term than London had employed at the start.

In the event, the piece he wrote during conference was about Ed Miliband, then Labour leader. The on-message article began: "Comrades, I have the solution. I have an export-led answer to Britain's current economic difficulties. We need to export Ed Miliband to China."

There was reason for the chancellor to worry about the mayor. In 2009, while the party was in opposition, Mr Johnson caused problems for his party's conference.

At the time, policy on Europe was being hotly debated. The official line was to be in favour of a referendum on the Lisbon Treaty - an EU agreement - before the treaty could come into force.

But the party view was that if the treaty had already taken effect by the time they got into power, there would be no referendum. This was controversial, and Mr Johnson whipped up division by suggesting that there should be a referendum in any event.

In an intervention that caused problems for the party throughout the week, he told the BBC at the start of that conference: "It's a difficult matter and obviously William Hague [then shadow foreign secretary] and David Cameron will have to give effect to the consultation I think people will want to have."

Taming the troublemaker

Mr Osborne and Mr Johnson's relationship will be closely watched in coming years. The two men are presumed to be contenders for the leadership of the Conservative party when David Cameron steps down during this parliament.

City Hall insiders see the episode as a victory for their man - he won extra money from the chancellor for the city ahead of the Olympics at no cost to the mayor.

Individuals close to the chancellor said, though, that the mayor had been elected on the basis that he could win a good deal for London. They wanted to support him by funding this request - and others.

He was, at the moment the email was sent, campaigning for re-election in 2012 and Mr Osborne, who moonlights as the Tories' lead strategist, was keen for him to hold the capital city.

Furthermore, the idea of further police funding was, in any case, not a hard political sell. The deal was struck shortly after the 2011 riots, when looting and arson broke out in city centres. It also came shortly before the London Olympics. The policing budget for that was a source of political controversy for several years. The deal worked for both men.

City Hall has declined to comment. The Treasury has been approached for comment.