For a split second yesterday, John Hutton's eyes bulged noticeably as the stark reality of "hammering" five million low-paid workers appeared to hit home.

Amid all the talk about increased tax credits and child benefit, and how Labour since coming to power had significantly helped the less well-off, it was put to Westminster's Business Secretary that a single person earning £7000 a year should be the last person to be asked to pay more in tax, and the minister's heart appeared to miss a beat.

Indeed, one could argue that the Labour government's heart has missed several beats over recent budgetary decisions, with the abolition of the 10p tax rate being the most contentious.

Sources at last week's comradely gathering of Labour MPs and peers were astonished at how frank people were about griping at their leader. One usually loyal back bencher pointed out that Labour was in danger of losing its core vote and complained: "Gordon isn't listening."

A large medley of the usual, the not-so-usual and the highly unusual suspects has signed Commons motions demanding the PM rethinks scrapping the 10p tax rate. However, it will not happen.

So why have fraternal voices become so shrill all of a sudden? Some back benchers complained months ago when, in his last Budget, Mr Brown performed his now notorious "con" trick. Cutting the basic rate from 22p and 20p might to some have appeared a good media wheeze at the time but, on closer inspection, it was paid for by saying farewell to the 10p rate, which has led to bitterness and bickering.

Yesterday, Nick Clegg, the LibDem leader, fumed at Mr Brown's "outrageous political stunt", arguing it was simply done to get a favourable headline in the next day's press. He insisted the poorest in society would pay the price. One could not help imagining many of Mr Brown's disgruntled colleagues nodding at the television set when Mr Clegg uttered those incisive words.

However, since the spring of 2007 when the tax change was proposed, the outlook for the world economy has taken a decidedly downward trajectory. Mortgage offers are shrinking, house prices are falling and inflation is set to increase. There are also forecasts of more job losses as companies pull in their horns.

Today, a report by members of the Labour-dominated Treasury Committee warns Alistair Darling that he might have given "insufficient weight" to the risks of continued global financial turbulence when making his forecasts for growth in the Budget. In other words, the road ahead might be bumpier than our prudent Chancellor envisages.

In his criticism of the UK Government, Mr Clegg also spoke of a "defining moment" having been reached, and one gets the sense that a watershed might have finally arrived in the life of the rather bruised-looking 11-year-old Labour government. With a fragile economy, bad news in England's local elections, continuing uncertainties in Iraq and Afghanistan as well as the still-to-be-played-out Labour donations saga with possible prosecutions, the future for Mr Brown looks decidedly bleak.

Talk, albeit sub rosa, has begun about whether the Labour leader will last the course. Premature perhaps, but intriguing nonetheless.

On Thursday, Messrs Brown and Darling will be hoping that the Bank of England will cast a welcome ray of sunshine into the economic gloom and cut interest rates. If it happens, it will be a slender silver lining on a very large dark cloud.