The Commerce Department in Washington has today hit the world with a series of revisions to how the world's largest economy performed in the three years since it technically emerged from recession in June 2009.

Any revisions raise questions about how accurately the data is capturing the performance of the economy. But, for a minute, let's accept that the US expanded at a much faster pace in the final three months of last year than first estimated. The Commerce Department now puts it at a 4.1pc annual clip.

It does not make the latest number the Commerce Department released today any more digestible. US growth in the second quarter slowed to 1.5pc from 1.9pc in the first three months of the year. Consumer spending weakened, business investment dropped and government spending fell.

Before wielding out apocalyptic headlines, it is worth emphasising that the economy is still managing growth that is eluding Britain and large parts of Europe. But that comparison will not provide any comfort to Americans, nor should it.

Significant progress on the path to recovery has been made since the recession. There has been major deleveraging of private debt, the housing market is stabilising in many parts of the country and the US is sitting on huge reserves of natural gas.

But today's figures underline that the recovery from the crisis is a painfully slow one that leaves the economy vulnerable to external shocks. Over the last two years, Europe has been more than generous in providing them.

As we head into the second half of the year, though, a domestic shock threatens. A series of scheduled tax rises and spending cuts are due to take effect at the start of the year, creating what Federal Reserve chairman Ben Bernanke has called a 'fiscal cliff' for the economy to fall off.

We cannot expect much clarity until after November's election on whether US politicians will steer the economy away from the danger. It is why an increasing number of economists are now expecting growth to slow this quarter and next.

It will then be up to whoever is in The White House and Congress to ensure that the US does not administer a shock at the start of next year.

Today's figures show the American economy is in no position to withstand it.