By Nick Assinder BBC News Online political correspondent

He has to show he is treating the allegations seriously - no matter what the strength of the claims.

Duncan Smith: Defiant defence

To dismiss them outright would only have led to claims of a whitewash and undermined his own standing and authority.

And the Tory leader's challenge to his anonymous detractors to "come after me" rather than target his wife will play well with Tory grassroots.

His bullish reaction to the claims have shown he is willing to fight them tooth and nail and may well bring him more support from party members.

But, despite his insistence that he wanted the probe, the decision is still a setback to Mr Duncan Smith at a particularly vulnerable time.

And if, at the end of it, he is criticised by Sir Philip then his already strained leadership will probably be over.

Of course, the fact that the allegations are to be investigated does not mean that there is substance to the claims.

Sir Philip was at pains to stress that one of the reasons he needed to undertake further investigations was that the claims were anonymous.

And if Mr Duncan Smith is cleared of any wrongdoing it could immediately remove any threat to his leadership.

Whatever the merits of the complaint, Mr Duncan Smith is fighting an increasingly bitter battle with the so-called "malevolent forces" within the party who he believes are out to destroy his leadership.

Tory splits

He is certain the latest row is simply another flank in that campaign by "cowards lurking in the shadows" and has robustly denied all the allegations.

He also clearly hopes the inquiry might just buy him some more time. And he is patently furious his critics have targeted his wife as a way of hitting at him.

But this could now drag on for weeks and will hang like a threatening thundercloud over everything he now does.

And it will make his job of reasserting his authority and heading off a leadership challenge far more difficult.

It is also certain to further lever open the splits at the heart of the Conservative machine that have been highlighted by the affair.

The danger for Mr Duncan Smith is that the wider party grassroots become dismayed by the apparent civil war and decide only a change of leader can put an end to it.

It certainly appears that, far from uniting after almost two years in-fighting, the party is out to tear itself to shreds.

In Westminster, the plotters continue to plot and backbenchers are trying to make up their minds whether they have to go through all the pain of a leadership challenge for what they believe may be a greater good.

Navel gazing

But there is no guarantee that such a challenge would end the party's woes.

It is quite possible it could make matters far worse - particularly if it resulted in yet another divisive leadership challenge.

Some believe such a period of pain may be cathartic and finally end the Tories lengthy period of destructive navel gazing.

The alternative view is that it would simply ensure the party had even less chance of winning the next general election.

It is also just possible that backbenchers decide the plotting has overstepped the mark and, as a result, rally around Mr Duncan Smith in a show of unity.

But that is not a habit the Tories been engaging in much of late.