Controversial Euro MP Roger Helmer has criticised moves to legalise gay marriage, claiming it is no different than siblings marrying.

Responding to comments made by the UK's most senior Catholic, Cardinal Keith O'Brien, who compared gay marriage to slavery in an article in the Sunday Telegraph, Helmer wrote in his blog: "If two men have a right to marry, how can we deny the same right to two siblings?"

Helmer, a member of the European parliament for the East Midlands, also wrote that O'Brien's original argument on gay marriage was "impressive and worth repeating".

"Once you start to tamper with the institution of marriage, you get into some very murky water indeed. If two men can be married, why not three men?"

The comments come just a few days after Helmer transferred from the Conservative Party to the UK Independence Party over a disagreement with the party's stance on immigration, climate change and Europe.

Helmer denied suggestions his comments were offensive to homosexuals.

"I've nothing against homosexuals, they can do what they like," he said. "But marriage has been recognised for thousands of years as a bond between a man and a woman. It's the best environment in which to raise children," he told the Leicester Mercury.