Mr Duncan Smith's authority as Conservative leader is now at stake

If Iain Duncan Smith is holding a bonfire night party for his children later today, I suspect that the effigy to be burnt will be that of John Bercow rather than Guy Fawkes. Mr Bercow's resignation from the Shadow Cabinet threatens to set off fireworks among Tory MPs, increasing the risk of the party leader's remaining authority going up in flames. It is certainly guaranteed to kick off a second consecutive week of parliamentary turmoil, making the gunpowder plot of 1605 seem a pretty tame affair by comparison.

As usual, it is the Tory party's confused attitude to unmarried parents and homosexuals which looks like tearing the party apart. David Willetts said at the Tory conference that "the war against single mothers is over". Meanwhile, Theresa May told the same gathering that they must no longer be seen as the nasty party. Yet on yesterday's Today programme she had to defend the appearance of being "nasty" towards gays, single parents and unmarried couples. Only two hours later, Michael Portillo was pointing out on Start the Week that "The Conservatives in this country are presently inconsistent. They do not maintain a message about their belief that people should be free to lead their private lives on their own terms. They don't deliver that consistency from one day to the next."

Meanwhile Mr Bercow's resignation ensures that the civil war in the party will plumb new depths. The tragedy is that any Tory reader of this column was given warning, on 22 May, when I suggested that it would have eased Tory tensions if a free vote had been allowed on the Adoption Bill during the Commons report stage.

I said then: "The Adoption Bill started out as a government Bill with all-party support and no mention of gay couples. Amendments were tabled and voted upon with the battle now going to the Lords. Tories there will no doubt use their majority to return the Bill to the Commons in the way it was originally drafted by the Government. The Government will then allow a good old ding-dong to occur before conceding the Bill in its original form – relieved that the gay adoption clause has been removed. Once again, however, the Tories may get bogged down on the issue." Yesterday's Independent reported that ministers are indeed preparing to ditch the proposals to allow unmarried and gays couples to adopt rather than abandon the Bill – but not before claiming a Tory scalp.

I also argued in May that it was perfectly reasonable to give Mr Duncan Smith the benefit of the doubt over his principal concern that the interests of children were more important than Labour backbench attempts to hijack the Adoption Bill to pursue a "gayer than thou" agenda. I continue to accept that it is debatable whether unmarried couples can provide the necessary stability to adopted children – and some Labour MPs voted with the Tories last time.

Mr Duncan Smith made a plausible case in his interview with Jonathan Dimbleby on Sunday that the children in question are damaged and vulnerable and need, even more than most, the stability of married parents. On this, he probably speaks for a majority of public opinion who might otherwise be liberal on the age of consent, Clause 28 and other gay issues. He certainly appeared to be strongly supported by the studio audience, which gave him a hard time on other issues. The Adoption Bill is about the welfare of children, and it should not be hijacked to promote pro-marriage or pro-gay agendas. But unfortunately for Mr Duncan Smith, anything that hinges on "gay" issues becomes a weathervane as to whether he is "inclusive".

At any rate, we now know that our wily Prime Minister is not going to go to the stake over the issue and invoke the Parliament Act to get the amended Bill passed. He wins every way: no gay adoption proposals but continuing turmoil inside the Tory party, a Shadow Cabinet resignation, with the added bonus that Mr Bercow will rejoin the troublemaking Portillistas, from whence he came prior to joining Mr Duncan Smith's team. Mr Blair knew all along that the Bill would be enacted in its original form, but was quite happy for Labour backbenchers to ensnare the Tories who, as usual, fell into the trap.

This time, however, the decision to impose another three-line whip is being seen as a wider test of Mr Duncan Smith's authority. In May, the party was still making steady, if slow, progress, and the events of the past three months, beginning with the sacking of David Davis as party chairman, had yet to cloud the political outlook. It was just possible to get away with the "soft" whip, and Mr Bercow was unwilling to rock the boat when the waters appeared relatively benign for the party leader.

The resignation of Mr Bercow at a time of turmoil has emboldened others to raise their profile on the adoption issue. There looked to be the prospect, at the time of writing, that the Tories would face in several directions in last night's vote. Andrew Lansley, a former member of William Hague's Shadow Cabinet, who took a principled position during the report stage in favour of adoption by gay and unmarried couples, is likely to vote along with Mr Bercow against his party. He will be joined by Michael Portillo, who abstained last time, while others will accept the invitation to absent themselves. It will be difficult to gauge how many are deliberate abstentions. It is clear, however, that the three-line whip is devoid of any meaning and will result in the Chief Whip having no authority to discipline any MP. Mrs May, the party chairman, was left floundering when she was asked what would happen to those who defied the whip.

This is the second week running the Tory MPs have been in crisis. After the brief respite provided by the Queen and her butler, and the attempts on the airwaves on Sunday by Mr Duncan Smith and David Davis to draw a line under last week's plotting, we are back to more bloodletting.

I suspect that if Mr Bercow had thought that the party was making progress on other fronts he would have bitten the bullet and gone off quietly, last night, to the cinema. Mr Bercow, of course, has to answer why he has resigned at this particular juncture. He did, after all, abstain on the "soft" three-line whip when the issue cropped up at report stage. Could it be that he has decided that the Duncan Smith ship is sinking and wants to be in a lifeboat before political lives are lost?

There would be one way of retrieving the situation, which might just get Mr Duncan Smith through this particular difficulty – the appointment to the Shadow Cabinet in a high-profile job of Alan Duncan, the gay junior foreign affairs spokesman who "came out" during the summer recess. This might just be the bold move, provided by Mr Bercow's resignation, to compensate for the botched reshuffle before the summer recess, and it could prevent further accusations of "nasty" and "homophobic". But in the meantime, the pressure on Mr Duncan Smith threatens to undermine everything he tried to achieve at last month's conference to look and sound nice. It all looks like turning very nasty.

mrbrown@pimlico.freeserve.co.uk