A Labour whip has apologised over a message on social media website Twitter which branded Conservatives "scum-sucking pigs".
But Telford MP David Wright insisted that his message must have been tampered with after he sent it out, saying he would not have used offensive language.
Mr Wright was one of a number of Labour MPs who used Twitter to respond to a poster campaign launched by David Cameron in which he tried to reach out to former Labour voters with the slogan "I've never voted Tory before but..."
A message went out on the Telford MP's Twitter feed reading: "ivenevervotedtory because you can put lipstick on a scum-sucking pig, but it's still a scum-sucking pig. And cos they would ruin Britain."
The message sparked anger among some other tweeters, including one using the name TrippyPip, who responded: "What a thoroughly objectionable turn of phrase you have - and you're an MP? People have actually voted for you? Shocking."
As a rash of furious tweets from Conservative supporters appeared on the 140-character-message site, a further comment went out on Mr Wright's feed, reading: "Oh dear, upsetting Tories again. Must've hit a nerve."
But the MP later sent out an apology, along with the toned-down comment: "IvenevervotedTory because you can put lipstick on a pig but it's still a pig. With apologies." At this point, the initial tweet was deleted from his account, but not before being copied by Tory bloggers.
In a personal message to TrippyPip, Mr Wright said: "Think you'll find the pig is the Tory Party not Tory voters. But fair does, apology in the ether."
And to another poster, going under the name BlackRodsSheep, he wrote: "Apologised. Not forced, genuine."
Mr Wright said that he had initially sent a Twitter message quoting Barack Obama's comment during the US presidential election campaign in 2008 that "you can put lipstick on a pig but it's still a pig".
But he denying using the phrase "scum-sucking", adding: "I wouldn't make derogatory remarks."
Asked how he explained the initial wording of the tweet, he said: "Somehow this has been adjusted, or somebody has tampered with it."
Mr Wright said he had issued an apology because people were offended by the Obama quote.
Conservative Party chairman Eric Pickles said: "This is exactly the sort of politics that voters are so sick of.
"This morning, David Cameron and I set out why people should vote Conservative for the first time.
"All one of Gordon Brown's ministers can provide in response is gutter language. This sort of language falls way short of what you'd expect of a minister of the Crown."