Image copyright Reuters

For years, columns have been written about it. Documentaries have been broadcast about it. Dinner parties have discussed it.

Panel shows have teased about it.

But today, has it actually happened?

It has already prompted the former Labour minister Angela Eagle to splutter: "What? Aghhhh."

Prime Minister Theresa May is currently off duty and on holiday, taking the alpine air in Switzerland.

Mrs May is known to have a love of Lucerne, the Bernese Oberland and the resort of Zermatt, with its views of the Matterhorn.

All of which means Whitehall's summer rota is up and running - a moment when opportunity knocks for a senior cabinet minister.

In previous years, the media have had hours of fun with the idea of John Prescott or Nick Clegg being left in charge of the country while their boss is sunning themselves in foreign climes.

'Silly season'

The main task of the summer stand-in is to stay out of the headlines and not cause any trouble, although the memo didn't reach John, now Lord Prescott, in 1997, when Tony Blair headed off to Tuscany for his first prime ministerial break.

The then deputy prime minister stunned onlookers at what should have been a routine photo opportunity launching a Thames flood protection scheme when he Christened a Chinese mitten crab floating around in a jar "Peter".

Image copyright Reuters Image caption John Prescott with 'Peter the crab'

His playful dig at friend and rival Peter, now Lord, Mandelson was "silly season" gold for tabloid hacks hungry for New Labour split stories.

Theresa May was the summer stand-in herself in 2013, along with the then Foreign Secretary William Hague, when David Cameron and Nick Clegg both decided to take a break at the same time.

Last week, as Mrs May headed to the Alps, it was Chancellor Philip Hammond who got the call.

Mr Hammond, affectionately nicknamed "Box Office Phil" by reporters, for his capacity for less than riveting public appearances, is seen as the very personification of a "safe pair of hands".

It's not a phrase used quite so often about Boris Johnson.

Yet the foreign secretary is the "senior minister on duty" in the UK.

'Standard practice'

Does that mean he is running the country? What is mobile reception like in the upper reaches of the Swiss Alps?

"The prime minister is very much in charge and is constantly kept abreast of what is going on," said a Downing Street spokesman.

Has Mrs May been on the phone to the foreign secretary?

"Not that I'm aware of," the spokesman added.

Asked to clarify what being "the senior minister on duty" actually meant, the prime minister's spokesman said: "I don't actually know. I'll try to find out."

An hour or so later, Downing Street told me: "It is standard practice for a senior duty minister to be appointed whilst the PM is travelling.

"They can attend to urgent business or meetings if required. The PM remains in charge and will be updated throughout."