A key aide of Michael Howard once stood as a candidate for the UK Independence Party, it has been revealed.

George Eustice, a press secretary to the Conservative leader, stood for the UKIP in the 1999 European elections, the Sunday Times reported.

The revelation prompted Labour criticism of Tory links to the anti-EU party.

And Eustice said he regretted the decision to back the UKIP, a party that was branded extremist in leaked internal Tory documents.

"With hindsight it is obviously something I regret - it was a mistake," he told the Sunday Times.

"Although there are quite a lot of well-meaning people involved in the UKIP they are quite seriously deluded."

Labour chairman Ian McCartney said the revelations "show the continued and close ties between the Conservative Party and UKIP".

"By his own definition, Michael Howard has appointed an 'extremist' to his own team and provides yet further evidence of the rise of a new Militant Tendency in Tory ranks," he said.

"The Tories are falling apart on Europe again because of Michael Howard's pandering to the anti-European extremists inside and outside his own party.

"The Tories' policy of disengagement leading to withdrawal would put British jobs, British trade and British prosperity at risk."

The latest developments came amid speculation that Robert Kilroy-Silk could become the leader of the UKIP.

He said on Sunday that the proposition had been put to him by other members of the party.