INVESTIGATING ministers is a tricky business, even for Cabinet secretaries.

A number have tripped up when looking at whether a senior politician has behaved properly.

That is why Sir Gus O’Donnell is taking a measured and unhurried approach to his inquiry into whether Tessa Jowell has broken the ministerial code. It is not just her future that is at stake. It is also his reputation.

Successive prime ministers have looked to their Cabinet secretaries to handle awkward issues, whether political memoirs or allegations of misconduct.

Lord Armstrong of Ilminster acted as the shield for Baroness Thatcher over both the Westland and Spycatcher affairs. Lord Butler of Brockwell was put in an impossible position and made to look naive because of Jonathan Aitken’s lies. Later, Lord Wilson of Dinton had to discuss Geoffrey Robinson’s future with him personally over the charges about his business interests. He also provided crucial advice during the hectic days leading to Peter Mandelson’s second resignation, though he did not himself conduct the inquiry into the Hinduja passports affair.

Sir Gus has learnt from these experiences, not least from his time at No 10 as press secretary to Sir John Major when sleaze allegations against Tory ministers first erupted. He has also decided to become more involved in the political side of his job than his predecessor, Lord Turnbull of Enfield, who concentrated on delivering reform of public services. So other permanent secretaries have turned to Sir Gus for a lead on the Prime Minister’s political mood and concerns.

Four months ago Sir Gus also played a crucial role in David Blunkett’s resignation for a second time. He cast himself as the guardian of the ministerial code, examining whether Mr Blunkett had breached it, and then making known that he had.

This did not mean that Sir Gus said whether Mr Blunkett should stay or resign: that is, of course, beyond his remit.

But Sir Gus’s public role made Mr Blunkett’s departure inevitable.

The same is potentially true now for Ms Jowell. If Sir Gus finds that she breached the code, which she has strongly and repeatedly denied, she will have to go. Sir Gus’s central role has also made Downing Street more cautious.

During the Blunkett affair, the Prime Minister’s spokesman declared his strong support for the Work and Pensions Secretary then, before Sir Gus had reported, giving the appearance of prejudging his conclusions.

This time, the official line has been more cautious and non-commital over the past two days. The Prime Minister does not want to get caught out again in offering premature support.

No 10 wants to see Sir Gus’s conclusions first.

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