WHATEVER you do, don’t say “Boris”. That was the instruction to cabinet ministers last week as the race for London mayor became even more heated.

In an unusual move, the most senior figures in government were told they would have to pay £5 into a “swear box” each time they referred to Boris Johnson, the Conservative mayoral candidate, by his first name alone.

Why? The government fears that calling him simply Boris makes the maverick appear too charismatic and popular among the voters. Instead, they must refer to him more formally either as “Boris Johnson” or the “Tory candidate”.

The swear box scheme was unveiled at a cabinet meeting on Tuesday by Tessa Jowell, the minister in charge of Labour’s campaign for London mayor, amid polls showing that Johnson is on course to win on May 1.

“She told us that if we refer to him as ‘Boris’ we will only be playing into the hands of the Tories,” said a minister.

Minutes after Jowell made the remarks, the swear box claimed its first victim. Offering his view of the Conservative challenge, Jack Straw, the justice secretary, absent-mindedly referred to “Boris” – and duly took a fiver out of his wallet as he was howled at by excited colleagues.

Johnson, who achieved fame on television shows such as Have I Got News for You, is one of only a handful of politicians who is on first-name terms with the electorate. One of the others is his rival, Ken Livingstone, the London mayor who is known in the capital simply as “Ken”.

These political showmen enjoy the sort of recognition which is usually the preserve of celebrities such as Kylie and Beyoncé. Although Gordon Brown is sometimes known as “Gordon”, it is often in a derogatory context, referring to the prime minister’s controlling instincts. At present David Cameron, the Tory leader, is only “Dave” to his Notting Hill acolytes.

It is not clear how the money from the swear box will be spent, nor how rigorously Jowell will enforce it.

It is part of a counter-at-tack by Labour chiefs who fear that the loss of London to the Tories would increase the risk of defeat at the next general election.

This weekend Livingstone, who is fighting for a third term as mayor, faces a series of disclosures about his personal life and campaign donations. They follow revelations last week that he has a son and two daughters by two previous girlfriends – as well as two children with his current companion, Emma Beal.

He said that the revelations would not damage his election prospects. “I don’t think anybody in this city will be shocked by what two consenting adults do, as long as you don’t include children, animals and vegetables,” he said.

“I’m quite happy with my private life and the people who have shared my private life over the last 40 years. No one has ever found anything in my private life that was illegal or immoral.”

Although Livingstone says he sees all his children on a regular basis, he has been less than forthcoming in public about his responsibilities as a father. The Sunday Times has obtained the birth certificate of the 15-year-old son Livingstone had with Janet Woolf, a teacher from north London.

The field on the form where the father’s name should appear is left blank.

When Livingstone had his first of two children with Beal in 2002, it was widely reported that he was becoming a father for the first time. The mayor of London did not clarify that this was not the case until last week, ahead of the publication of a biography that reveals his family life.

It has also emerged that Livingstone’s election campaign has received £60,000 from trade unions linked to recent industrial disputes on the London Underground.

Aslef, which has staged a series of strikes on commuter lines over the past two years, has given £10,000 to his campaign.

The mayor is also thought to have received a £50,000 donation from Unite, which last year held the Tube to ransom by threatening strike action – before backing down just hours before the walk-out was due to begin.

Officially, Livingstone’s backers give funds to the Labour party, which are then passed on to the mayoral campaign. This legally allows the identities of his donors to remain secret until after the May election.

The mayor has claimed that he does not know who is donating to his reelection campaign.

Both unions have claimed they are giving directly to Livingstone rather than to the Labour party. One Aslef official said: “Yes, we’ve given to Ken.”

Unite’s donation is revealed in the minutes of a meeting of the union’s parliamentary group six weeks ago. The document reads: “£50k will be donated to Ken Livingstone’s mayoral election campaign.”

Tessa’s swear box

Cripes! Tessa Jowell, below, has decreed Labour fi gures should fork out £5 every time they refer to Boris Johnson by his fi rst name

Ken Livingstone £10

‘I can promise Boris and I have never slept together. He is not my type’

‘As soon as I saw the poll which showed I was neck and neck with Boris I thought this is going to get dirty’

Hazel Blears £5

‘Boris is no joke. He’s a nasty right-wing elitist with odious views’

Ed Balls £5

‘Boris would be driving me up the wall or up a tree [if stranded on a desert island with him]’

Video of the week: the Boris clip everyone must watch

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