Will he or won't he? Should he or shouldn't he? If he does, what will it say about him, David Cameron and the prospects of the Tory party? The interesting thing about the latest burst of reports that Boris Johnson is planning to stand as an MP at the next election is that he has remained mute. The normally voluble mayor, a man rarely caught short of an opinion about anything, has been Trappist about his own future.

It is certainly being noted by people around David Cameron who keep a constant and wary eye on the blond over the water. "He's doing nothing to damp it down," notes one friend of the prime minister. "There's been this huge amount of speculation and it is notable that he has not come out and said, 'I'm not standing."'

No, he has not and the reason why he hasn't is obvious. There is turmoil in Borisworld. Both his admirers and the man himself have been agonising for months about whether his ambitions are best advanced by plotting a return to the Commons next May or by waiting until his mayoral term is complete in 2016.

Toryland is divided on the subject. Paul Goodman, the editor of the ConservativeHome website, urges him to stand. James Forsyth, the political editor of the Spectator, warns that it would be a mistake. That split among the Tory commentators reflects the wider division within the Conservative party.

Then there are those Tories who are simply becoming tired with and frustrated by the whole soap opera, a feeling you might well share. As we report today, influential figures on the executive of the Tory backbench 1922 committee reckon he needs to make his mind up and quick because the incessant speculation about his intentions is destabilising.

The reason it is such a hot topic among Tories is not just because he is a big player, their most popular politician and a potential leader. It is also because his decision will send a significant message about what he really thinks about the election prospects of his fellow old Etonian at Number 10.

There is no constitutional impediment to prevent him from returning as an MP in 2015 and continuing to sit in City Hall for a further 12 months after that. Many will recall that, after Ken Livingstone's election as mayor in 2000, he continued to be the MP for Brent East until the general election a year later. Though not strictly the same – that was Ken's first year of his first term in a newly created post – Boris could and no doubt would sort of argue that this sort of set a sort of precedent. Adored as he is by many Tory activists, he would have no trouble finding a seat. There is already a clamour from some Conservative associations to have him as their candidate.

But there are big risks for him and that is what explains his dithering. The largest of those risks is that he would do serious damage to his reputation by breaking his promise not to seek to return to the Commons before his mayoral term is up in May 2016. As recently as last month, when asked by LBC's Nick Ferrari whether he was preparing to stand as an MP, the mayor replied: "The daily excitement of helping to run the greatest city on Earth, why would you want to do anything else?" He went on: "I'm sticking to the job that I was elected to do in 2012."

Some note that he left himself a bit of wriggle-room: while the spirit of those remarks suggested that he wouldn't stand as an MP, he did not actually make an unequivocal statement to that effect.

However, I can find no escape clauses at all in the declaration he made during the last mayoral campaign to the London Evening Standard. It is worth quoting in full: "If I am fortunate enough to win I will need four years to deliver what I have promised. And having put trust at the heart of this election, I would serve out that term in full.

"I made a solemn vow to Londoners to lead them out of recession, bring down crime and deliver the growth, investment and jobs that this city so desperately needs. Keeping that promise cannot be combined with any other political capacity."

Not just any old political promise, then, but a "solemn vow", the political equivalent of swearing on the Bible, not to seek any other elected office for the duration of his term as mayor and a holy pledge couched in terms of "trust".

Maybe he could betray that promise and not suffer too much adverse reaction. This is the view taken by many of his fans among Tories. One Boris-friendly Conservative recently told me that he didn't think "most voters would be all that fussed really" because everyone is well-aware that he is entering the final stretch of his time at City Hall and his interest in becoming prime minister is not exactly a secret.

You might take a view that people are so accustomed to politicians breaking promises that they would merely respond with a weary shrug. You could also think that whatever appeals to voters about Boris, his brand has not been built on reliability anyway. This is the man who once said that his policy on cake was to be "pro-having it and pro-eating it".

The flings, the fibs, the flip-flops, he has survived, even thrived on, a variety of misadventures and scandals that would have felled more conventional politicians. Those who have conducted focus groups about him report that participants are aware that he has often got into trouble, but give him a benefit of the doubt that other politicians would kill for and are inclined to put a forgiving interpretation on the controversial episodes of his career. That helped him get elected, and then re-elected, in a traditionally Labour city.

The normal rules of politics don't appear to apply to Boris. But it would nevertheless be quite a gamble to assume that this would continue to be the case if he were to flagrantly break a "solemn vow".

One of his attractions to voters – one of the things that has made him one of those politicians who can ride the anti-political mood – is that he comes over as more authentic than most. Breaking his promise to eight million Londoners in pursuit of personal ambition would put that at risk.

Labour would relentlessly hammer him for it and say that he was a part-time mayor sacrificing the interests of the capital on the altar of his ego.

Some Tory strategists are sceptical that he would get away with it. As one senior adviser at Number 10 puts it: "The rules don't apply until they do."

What will also be troubling him – or certainly should be – is that he will know that none of the excuses he might advance will pass even the most cursory of inspections. I expect he would try to justify breaking the promise by claiming that being an MP would allow him to represent London in parliament. I doubt there will be many buyers for that defence and especially not if he pursues a seat such as North West Hampshire or Louth and Horncastle, to name two of the out-of-capital constituencies being touted as potential parliamentary berths. One of his talents as mayor has been championing the capital and he's done it well for six years without the need to have a perch on the green leather benches of the Commons.

If he decides to declare that he is standing as an MP, another argument we will hear will be that the Tories need him for the general election campaign. That claim is already being made by David Cameron who said last week that his rival was "a great striker" whom he was keen to have "on the team". The prime minister feels obliged to grit his teeth and say that he wants Boris back, but I'm not at all convinced that the Tory leader really hopes to see the man who is after his job in the Commons in 2015. There is a false note when David Cameron speaks in footballing metaphors.

Mayor Johnson doesn't need to stand as an MP to be on the election campaign trail. He already has a national recognition factor the equal of any member of the cabinet; he can already command vast amounts of media attention; he is already supposed to be on the Tory team. Standing as an MP would add exactly nothing to his capacity to play a prominent role in Conservative campaigning.

When you have dispensed with all the bogus excuses for breaking his "solemn vow", that leaves only one real reason for doing it: not because he wants to help the Tories to win, but because he calculates that it is quite likely that they won't. The only rational purpose for rushing back to the Commons is because he thinks there is a good probability that there will be a vacancy for the party leadership after the election. Whatever he or David Cameron say to the contrary, even the dogs on the streets know it.