The remarks, by Austin Mitchell, came as Pfizer executives appeared in parliament for a second day to answer questions from lawmakers over a potential takeover bid that has fuelled a debate about whether the government should block deals to protect British firms.
Prime Minister David Cameron said on Wednesday he was looking for the best possible commitments from Pfizer on continuing investment in Britain, in response to criticism from Labour that his government was putting business interests before the national interest.
"Cameron dare not stop Pfizer because he dare not offend the US (United States) in any way. Roll up rapists," Mitchell posted on social networking site Twitter.
Cameron's spokesman said Mitchell's analogy was "quite wrong", while Nicky Morgan, the government's minister for women, wrote to Mitchell's boss, Labour leader Ed Miliband, to demand an apology and a retraction.
"People across the whole country will be appalled that anybody would try to link a proposed corporate takeover with the horrific crime of rape," Morgan said in the letter released by Cameron's Conservative party.
Mitchell defended his choice of language in a subsequent tweet, saying that the word "rape" was often used in other contexts, for example to describe the destruction of rainforests.