It’s always the economy, but not always in the same way. The most important event in the real – as opposed to the political – world this month to affect Gordon Brown’s election decision is the turbulence in financial markets.

Go back little more than four weeks and the outlook was pretty benign. Admittedly, inflation and interest rates had been rising, but both seemed to be near, if not yet at, their peaks. The growth of consumer spending and housing activity was slowing. But Mr Brown’s much-hyped expansion – which started well before 1997 – looked set to continue for another record year. So Alistair Darling was plausibly able to claim that the economy was “in good nick”, “strong and stable”.

Since then, we have had the gyrations in global financial markets. The message from Mr Darling and the Government has been don’t panic. That is sensible, even though credit may become tighter here as lenders become more cautious.

Yet there are risks, not just the more alarmist ones of recession but also of a reduction in stock and housing market activity. Treasury finances have become increasingly dependent on the level of share and other asset prices and the amount of trading. For example, the share declines in 2003 produced a shortfall of corporation tax of more than £2 billion the following year. And receipts from stamp duty on share trading and house sales have proved to be volatile, moving up and down by more than £2 billion to £3 billion between years.

So far, the negative impact of the recent volatility appears relatively small, according to the Institute for Fiscal Studies and other analysts. This is because the weakness of the markets came after very strong growth in the preceding months.

Leaving aside the detailed figures, these uncertainties could favour Mr Brown and Mr Darling. If voters are worried about the economy, they are more likely to opt for the safe and trusted than the unknown and unproven. Mr Brown – Mr Prudent – and Mr Darling, the epitome of caution, could look more reassuring than David Cameron and George Osborne.

However, many families will face a big rise in their mortgage payments this autumn at the expiry of two-year fixed-rate deals arranged at much lower rates.

Otherwise, the main news will be the comprehensive spending review (CSR). Both the overall rate of growth – 2 per cent a year in real terms up to 2010-11 – and most departmental budgets are known. The main plans still to be unveiled are for health, transport and international development. Mr Darling is due to announce these in his combined CSR and PreBudget Report in mid-October. Given Labour’s problems over the NHS, its main weakness with voters, Mr Brown could make an announcement about health spending the centrepiece of his party conference speech next month.

Overall, what matters for election timing will not just be the state of the economy this autumn, but whether it looks better or worse next spring. At present, the odds are against an early election. But speculation will come and go for another month, in line with the latest opinion polls.

Times Recommends

• Time the law caught up with over-65s
• Don’t fear rate rises, fear stagflation
• I could be a celebrity – get me out now!