By Nick Assinder BBC News Online political correspondent

There has never been any formal mechanism for debating the role of a prime minister's spouse.

Such a suggestion would have been seen as impractical, unnecessary and even faintly insulting.

But there has been an unwritten understanding that husbands and wives steer well clear of anything overtly political or controversial.

Hilary had political ambitions

It has all been a matter of common sense and an awareness of the obvious sensitivities of not dropping your other half in it, or stealing their thunder.

Get a political animal in the role, however, and things were always likely to get sticky.

Political role

And - like Hilary Clinton, but unlike virtually all previous prime minister's spouses - Cherie Blair is most definitely a political animal.

She has strong political beliefs, with the regular jibe being she is the only Blair with strong political beliefs.

And it is almost inevitable they are going to show themselves from time to time.

Glenys is successful MEP

Also like Hilary Clinton, she has been given a role inside the government machine. With Hilary it was developing health strategy, with Cherie it is chairing Downing Street policy seminars.

But, in Britain, there have never before been cases of the prime minister's spouse stepping into the political limelight.

Private views

Things may well have been different if Neil Kinnock or Michael Foot had ever become prime minister.

Both their wives were deeply political individuals with strong beliefs of their own.

Glenys Kinnock currently has a successful career as a leading Labour Euro MP.

Before them, neither Audrey Callaghan or Mary Wilson overtly strayed into political territory.

Even privately they were careful to ensure they did not speak out on sensitive issues when there was any chance of their comments being reported.

Before that, it was not an issue. Wives - because they all were wives, of course - simply did not have a profile.

One of the first spouses to attract great media interest was, inevitably, the husband of the first woman prime minister.

And Dennis Thatcher was clearly an outspoken individual with his own beliefs.

But, with the help of Private Eye, he carved out a clever image for himself as the amiable, thirsty buffer who deferred to "the boss".

Off the radar

He played his golf, enjoyed his gin and tonics, ran his companies and occasionally amused the media with his asides.

Ffion kept in background

But he left the politics to the boss.

Norma Major's profile was so low as to be virtually off the media radar altogether.

She loved opera, and wrote books about it, but, it was said, disliked the political lifestyle.

In the early days she was happy to be portrayed as the loyal housewife and home maker.

That was always a bit of a caricature, but there was never any fear of her joining the political debate.

Ffion Hague, a successful career woman in her own right, made the deliberate decision to accompany her husband where necessary but to play no other part in politics.

Cherie is, without doubt, a different case.

Whether she has her own political ambitions or not will continue to be a matter for media interest.

And her public appearances and utterances will now be crawled over like never before.

It is not, therefore, surprising that there are the first calls for the role of the prime minister's wife to be defined more clearly.