Allegations that Labour has 'cynically' targeted cancer sufferers with an election leaflet are being investigated by the Information Commissioner, Christopher Graham.
The Commissioner's office said it had called Labour officials to find out the facts after protests were reported by cancer patients who received Labour election postcards warning them that the Tories would cut cancer services if David Cameron won power on 6 May.
Even if Labour is cleared of breaking the data protection laws the exercise has already backfired on the party's attempt to emulate the use of 'micro-targeting' of voters used by Barrack Obama's camp in the US presidential elections.
Shadow Cabinet Minister Liam Fox said Labour should be "ashamed of themselves" for peddling "the politics of fear".
"It is one of the most cynical exercises in recent British politics and the Labour Party should immediately distance themselves from it," Mr Fox said. "Patients who suffer from life-threatening illness and their families have enough to deal with. Labour should be ashamed of themselves."
Labour sent out 250,000 postcards to voters, including some who had received treatment for cancer within the past five years, warning a Labour guarantee to see a cancer specialist within two weeks would be scrapped by the Conservatives. They carried a warning: "Are the Tories a change you can afford?"
The Tories claimed that, in the Poplar and Limehouse parliamentary constituency, a woman who had a potentially cancerous lump was the only person who received a 'cancer' postcard from the Labour party among 50 neighbours. One card was even delivered to a woman who had recently died of breast cancer.
Phyllis Delik, an 80-year-old cancer patient in Nottingham, who received one of the Labour cards through the post described it as 'despicable'.
"It is obvious I was targeted because of my medical history. Why else were we the only ones to received these cards, personally addressed to us?" said Mrs Delik.
A spokesman for Mr Graham said: "The Information Commissioner's Office is seeking clarification from the Labour Party about the mechanism used to determine who this leaflet was sent to."
Labour officials said it used files from a private firm to target voters, using information stored with their permission.
Gordon Brown insisted the Labour Party had no knowledge of the health details of voters after campaign leaflets on cancer policy were sent to people who suffered from the disease. He said "no personal health information" about any voter was known to the party.