David Cameron has called on the Prime Minister to apologise for sending campaign leaflets on cancer policy to people who suffered from the disease, denouncing Labour's tactics as "sick".
Labour strongly denied that the postcards were specifically targeted at cancer sufferers but Tory leader Mr Cameron said many recipients had been "very angry and very upset".
Personalised cards were sent to 250,000 women last month, warning that a Conservative government would scrap a Labour guarantee that all suspected breast cancer patients would be seen by a specialist within two weeks of GP referral.
The Sunday Times reported that some women who received the cards were cancer patients or had previously suffered from the illness.
In an interview with the Metro newspaper, Mr Cameron said: "I think this is just an absolutely appalling way to behave. If Gordon Brown has a moral compass he should get it out and have a good look at it and apologise to these people straight away."
But Labour denied targeting patients and said that cancer was sadly so prevalent in society that some recipients were likely to have suffered from it.
Health Secretary Andy Burnham said: "It is wholly wrong to suggest that the Labour Party has, or would ever, target cancer sufferers with its leaflets and I totally reject that claim. The real issue here is the choice that the country faces on the NHS.
"Whilst Labour will guarantee rapid access to cancer specialists and cancer tests, the Conservatives have repeatedly said they would scrap those guarantees, despite all the evidence which shows that they save thousands of lives and will save thousands more lives in the future."
Liberal Democrat Treasury spokesman Vince Cable - whose first wife died after a long battle with cancer - said it would be "disreputable" if databases had been used to target the message at sufferers, and demanded an inquiry to determine whether this had happened.
"There needs to be proper investigation to find out if there has been abuse. I feel strongly about this because my late wife died of cancer. We lived with this for 14-15 years, and if we'd received cards on an individual basis trying to make party politics out of it we'd have been very angry indeed."