PETER HAIN last night demanded an apology from Tory leader Michael Howard for "misleading" Welsh voters.
The Welsh Secretary accused Mr Howard of running a dishonest campaign of "sharp practices".
He said the Tories had exaggerated claims about hospital superbug rates in Wales.
The MRSA row flared up last night - a week after Mr Howard was forced to apologise for employing similar tactics in England.
In a campaign letter, Mr Howard claimed 125 people had contracted MRSA in just one hospital trust in Wales. The figure was five times the actual amount.
Mr Hain said, "This is another example of the Tories' dishonest campaigning. Mr Howard should apologise and stop these sharp practices. They've been caught out in this way before and had to backtrack. Nobody in Wales can trust the Tories.
"They should stop playing dirty and start facing the facts. Meanwhile we'll tell the truth about the Tories and their record."
The Conservative leader was forced to say sorry last week when it emerged that campaign letters and leaflets circulating in England contained exaggerated figures of hospital superbug infections.
Welsh Labour believe the same tactic has been employed across Wales after getting hold of letters and leaflets sent to voters in Conwy. Signed by Michael Howard, the letters claim 125 people contracted MRSA in the Conwy NHS Trust.
Figures published by the National Assembly in February show the number of cases in Conwy was actually 25 in the 12 months to March, 2004.
Betty Williams, Labour's parliamentary candidate in Conwy, has written to Mr Howard demanding a retraction and apology.
"Even the number for the whole of North Wales was far short of the figure claimed by Michael Howard," she said. "It is staggering that the Tories are attempting to mislead people over MRSA when their record is so lamentable."
She said she had demanded that Mr Howard apologise to the people of Conwy for misleading them, insisting "this irresponsible scaremongering cannot be allowed to go unchallenged."
Shadow Health Secretary Andrew Lansley told the Western Mail, "The figure reflects accurately the number of bloodstream infection rates for the area. The letter should have read "local NHS Trusts" but unfortunately read "Trust".
"MRSA has become endemic in our hospitals deaths associated with MRSA have tripled since 1997. The issue, of critical importance to NHS staff and patients, has not been a priority for Labour. In contrast the Conservatives have set out an action plan to deliver what matters to people - clean hospitals."
A week ago Tony Blair accused Mr Howard of running a "nasty and unscrupulous" campaign after the Conservatives admitted producing misleading leaflets on MRSA like those sent out in Wales.
In North Yorkshire, people were told there had been 247 cases of the superbug in a year at a local hospital when there had been six.
In Twickenham, south-west London, voters received letters saying there had been 166 MRSA cases but the two local hospitals had only 60 in 12 months.
Mr Howard apologised for the error but added, " I'm angry about the 5,000 people who die every year from hospital-acquired infections in our country, that's what I'm angry about."
He said the number of people who died from hospital acquired infections had risen under Labour and laid out Tory plans to put matrons back in hospitals with the power to close infected wards.