Simon English: Clarke's move could see him carry the can

Outlook: One of Warren Buffett's stock market aphorisms is that you should invest in companies any idiot could run, because eventually one will. Mr Buffett owns 5 per cent of Tesco and may be beginning to wonder if Phil Clarke is that man.

That would be a premature judgement to say the least, but the new (ish) chief executive has clearly got troubles in store.

His move to take charge of the UK operations, ousting Richard Brasher in the process, is supposed to look bold. There's only one boss and I'm it, he's saying.

It's brave in the sense that it leaves Mr Clarke with no one else to blame should Tesco's market share keep sliding. But it leaves him with an almighty job, doing the work of at least two – perhaps three – highly committed executives. There's some City scepticism that this is wise or sustainable.

Mr Brasher is now the third highly regarded director to leave since Mr Clarke succeeded Sir Terry Leahy a year ago (Andrew Higginson and David Potts were the first two).

It might have been a necessary shake-up, it might have happened anyway, but it does begin to look bad. How many more departures can Mr Clarke preside over before investors get nervous?

For any other board member feeling a bit irked, now's the time to march into Mr Clarke's office and threaten to quit unless demands are met.

And it isn't just in the UK supermarket business that Tesco has issues. The US business still looks like a misadventure (Tesco vigorously denies this) and the bank still has teething problems.

There's also shifting patterns in how and where people shop.

Charlie Mayfield of John Lewis Partnership – he could plausibly claim to be the best retailer in Britain if he were the bragging type – thinks there are already too many supermarkets and that building more is madness.

Tesco is building more. It would be extraordinary if Tesco, from a position of such huge, market strength, made a hash of it. It is possible.