A row has broken out over Conservative claims that Labour had not lived up to a promise of free educational visits to Auschwitz concentration camp.
The Holocaust Education Trust organises trips to Auschwitz
The Conservatives included the Labour pledge in a list of 26 "gimmicks" it said the government failed to deliver.
Labour hit back by saying the reference to Auschwitz visits as a "gimmick" was "truly disgraceful" and offensive to the Jewish community.
But the Tories said there was genuine concern about the lack of funding.
The reference to Auschwitz visits was in briefing notes to a speech by Conservative leader David Cameron.
It said the government has promised funding for two pupils from every sixth form and college in the country to visit the Nazi concentration camp where millions of Jews were murdered during World War II.
But schools must find £100 towards the £350 cost of every sixth formers' trip.
A Conservative spokesman said: "There is no way we think that trips to Auschwitz are a gimmick.
"The gimmick is for the government to make an announcement without providing the means for it to happen."
But Schools Secretary Ed Balls said: "Anyone who has seen the horrors of Auschwitz at first-hand knows what a life-changing experience it is.
"To call the announcement I made of £4.65m to fund visits by school children over the next three years a 'gimmick' just beggars belief.
"In trying to make this issue into a matter of party politics, David Cameron has shown once again that he not only lacks judgement but also a basic sense of decency."
Schools had always been asked to come up with £100 towards each trip so that they were contributing to the experience, a Labour Party spokesman added.
In a statement, the Holocaust Educational Trust said it did not want to get involved in "party politics" over the row.
It followed a call by Labour peer Lord Janner, who is chairman of the HET, for Mr Cameron to retract the claim.
He said: "The Jewish community will be deeply hurt and offended by this.
"It's a low form of politics which David Cameron must already be regretting. I think it's very, very important that he apologises."
In a short speech, Mr Cameron attacked Mr Brown for being "so obsessed with short-term gimmicks".
"We've had a gimmick for every week that Gordon Brown has been prime minister," Mr Cameron said in Bolton.
"It would be funny if it wasn't so serious."
But Lord Hunt of Wirral, a Conservative peer and vice-president of the HET, defended Mr Cameron, saying there had been private concern over the government's funding promises, particularly when it emerged the trips would not be available to children in Scotland and Wales.
He said: "I greatly regret the Labour Party is seeking to making party political points out of what has been an all-party approach to Holocaust education and has the support of David Cameron.
"I have been raising concern within the Holocaust Educational Trust because there are conditions about how much it has to raise to put alongside the government grant.
"Now, when the small print emerges we're being told it doesn't apply to Scotland and Wales, we're being told that we will have to raise money within HET so it isn't quite what it appeared."
The HET organises one-day visits to Auschwitz followed by follow-up seminars. These are aimed at showing "what can happen if prejudice and racism become acceptable".
The Conservatives said that under a Conservative government the trips would be "funded in full".