Whatever happened to Iris Robinson? Within a few years she has been transformed from her husband Peter’s greatest asset and a formidable political force in her own right into his Achilles heel and an unguided missile who has party handlers pulling their hair out in despair.

Last Wednesday, the Robinsons departed these shores for their annual holiday in the US where they will, no doubt, spend some time considering the transformation in her image since devolution. People are openly comparing her to Ian Paisley Junior, who went from being his father’s right-hand man to a liability because of his ill-considered comments and business relationships once Paisley senior became Northern Ireland’s first minister.

Right now, it is Iris Robinson, and not Sinn Fein or the hard-line Traditional Unionist Voice breakaway group, who poses the biggest threat to the DUP’s growth and its acceptance in mainstream British politics.

“She has been digging this hole for about a month,” a senior British Conservative source said shortly after his party announced plans to merge with the Ulster Unionists.

The link-up was a shock to many in the DUP, who held talks with Tory chief David Cameron about possible co-operation in the House of Commons on Tuesday. That was just two days before Cameron and Sir Reg Empey, the UUP leader, announced they were setting up a working party aiming at unity.

On the face of it, the DUP, who have nine MPs to the UUP’s one, would seem a more tempting partner, but the image projected by MPs such as Robinson is just not British enough. Her grandstanding to the religious right is seen as alien and contrary to Britain’s image as a modern, multicultural and largely secular state.

Robinson’s recent claims that “the government has a responsibility to uphold God’s laws” does not ring true in a country where only 53% of people still say they are Christians, just 38% say they believe in God and many citizens are Muslim or Hindu.

Robinson’s views seem ill-thought-out. For instance, passing the Ten Commandments into law would mean legal sanctions against swearing and covetousness as well as the suppression of non-Christian religions.

A senior Tory source who was involved in the negotiations said: “Without being too unkind, the DUP are still more extreme than the UUP. Iris Robinson keeps reminding us of the DUP’s fundamentalist past.

“If you are David Cameron and you have spent years presenting the Conservatives as a modern, tolerant, inclusive force, nothing could be more damaging than a deal with somebody who makes comments such as she has.”

Iris has also shown what many see as a bitter and triumphalist tendency to gloat. Last month the DUP was wooed by the Conservatives, who sought their backing in a crucial vote to give 42-day detention powers to the police. It could conceivably have brought Gordon Brown’s Labour government down, but at the last moment the DUP came to his rescue with the nine votes he needed to win.

“After the vote was taken Iris held up nine fingers towards their benches and it was like a red rag to a bull. They kept mentioning it to me,” said Ulster Unionist leader Sir Reg Empey. “She was quite gratuitously adding salt to the wounds,” a Tory source said.

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