The increasingly bitter election campaign is continuing its downward spiral amid a row over the use of 'dirty tricks'.

Labour and the Conservatives were on Tuesday continuing a bitter exchange of insults and accusations.

The latest row focused on media reports, hotly denied by the party, that Labour is engaged in undercover moves to undermine the Conservatives.

A Sunday Times report highlighted a belief among Tory officials that their opponents might be mounting a series of undercover exercises as part of an "Operation Blackwatch".

Speaking at a press conference on Tuesday, Labour election chief Alan Milburn said he had no knowledge of anything called Operation Blackwatch, casting doubt on the accuracy of the report.

While not unexpected, Milburn's statement raises questions about whether the Tory claim was a ruse designed to distract attention from Michael Howard's own problems over his handling of the Howard Flight case

The leaking of the former Tory MP's unguarded comments pushed claims of 'dirty tricks' back to the top of the political agenda.

London's Evening Standard newspaper said that Tory officials had identified a potential "Labour mole" who attended the meeting at which Flight's comments were made.

But Milburn sought to distance his party from any involvement in the leaking to the Times.

"I think the Howard Flight revelations and how that came about, the source of that, is really a matter for the Times newspaper and not one for us," he said.

"The first I knew of it was when we got the report that it was going to be in the Times newspaper."

He also warned that claims and counter-claims over secret tape recordings were part of a "game" to distract attention from the "real issues".

"There is a game being played which the Conservatives are very keen to encourage, which is to get on to this issue of process and who did what and when at a public meeting," Milburn said.

"The issue for the Conservatives is a very straight forward one. Mr Flight has let the cat out of the bag. He has confirmed that they have big spending cuts planned."

But stoking the questions over the Tory chief's leadership, Milburn said he "notes with a great deal of interest" the criticism coming from some Conservatives who suggested Flight's expulsion may have been too severe a punishment.

The latest exchanges are just the latest in a series of increasingly personal and vitriolic attacks during the "pre-election" push.

Labour's campaign got off to a shaky start when posters depicting the Tory leader as a pig and as a Fagin-like character provoked accusations of anti-Semitism.

Meanwhile, a Labour minister accused the Tories of "bigotry" over plans to tackle unauthorised accommodation developments by Travellers.

Labour MP Kevin McNamara said the policy had the "whiff of the gas chamber about it".

Meanwhile, the Conservatives branded an attack on its health policies as "another Labour lie".

With the potential May 5 polling day still over a month a way, one of the longest general election campaigns in recent history could also go down in history as one of the nastiest.