Tory leader David Cameron's decision to vote for an immediate inquiry into the Iraq war was "absolutely crazy", a former party defence spokesman says.
Quentin Davies, who abstained from Tuesday's vote, said Tory credibility would be damaged if people thought the party had done a U-turn on the war.
Tory MPs were told at the last minute to back an unsuccessful SNP and Plaid Cymru motion for an immediate inquiry.
The Conservatives had been calling for an inquiry in about a year's time.
The SNP and Plaid motion called for a committee of seven senior MPs to review "the way in which the responsibilities of government were discharged in relation to Iraq".
The motion, which was backed by 12 Labour MPs, was defeated by 25 votes in the Commons.
Mr Davies was not the only Conservative to abstain, but he is the first to speak out publicly against Mr Cameron's decision.
He told BBC Radio 4's Week in Westminster: "I was quite incredulous when I heard we were going to vote for that [SNP/Plaid] resolution."
The Commons motion called for an immediate inquiry
He said he could not understand why anyone, especially his party, would vote for it.
"I didn't vote for it but most of the party did. [That] left me really quite amazed and I'm very sorry about that as a matter of fact."
He said it would be unprecedented to hold an inquiry during a military campaign.
To have an open inquiry would be "signalling to the enemy all your plans and all your weaknesses", he said.
"That can't make any sense at all, and I think that soldiers serving out there in these very difficult conditions would have been just as amazed as I was."
He said the party should be careful to base its actions on "analysis of national interest" and not to give the "slightest sliver of suspicion" that it was playing "party politics with these serious issues".
But John Maples, former shadow foreign secretary and ex-shadow defence secretary, said backing the Welsh motion was the "only way" open to the party to call on the government for an inquiry at some point.
He said while he agreed it would not be appropriate to inquire into current military operations while they were going on, the House of Commons motions had to be either supported or not.
Mr Maples said the party had supported the war on the basis of the Iraq dossier "which has turned out to be untrue", and they did not realise how "incompetently the occupation would be run afterwards".
"The facts have changed, it's quite right for the Conservative party to change its mind. I personally think it should have become more critical of the invasion and the post-invasion administration of Iraq sometime ago," he said.
But Mr Davies added the credibility of the party as an alternative government would be seriously damaged if it gave the impression of "cynically" shifting with the prevailing party political wind.
Another senior Conservative who was absent from the crucial vote - but who declined to be named - described Mr Cameron's decision as "intellectually and morally indefensible".
The MP suggested a number of Tory MPs were deliberately absent.
The Tories supported the SNP/Plaid motion after the government said it would not support their own call for an inquiry within the next year.
The Conservatives favour an investigation similar to the Franks inquiry carried out after the Falklands war, involving former generals and civil servants rather than just MPs.
They have said it should take place in about a year, by which time ministers have said Iraqi security forces should be taking over responsibilities from UK troops.
There are suggestions of tensions within the shadow cabinet over the decision to back SNP/Plaid - with claims it was shadow foreign secretary William Hague who pushed for the new line.