Nick Clegg reveals his 'non-secret weapon'

Mr Clegg said he married Miriam Gonzales Durantez for "for love, not three quid a week" as she made her first public appearance by his side since the May 6 election was called.

The Liberal Democrats also waded into the row over business leaders' support for the Tories' planned national insurance cut, describing their behaviour as "nauseating".

Unlike Samantha Cameron and Sarah Brown, Ms Gonzalez Durantez had shunned the limelight before appearing alongside her husband at a farm in his Sheffield Hallam constituency.

She looked relaxed in front of the cameras as she posed with her husband for photographs at the Whirlow Hall Farm Trust, a short walk from their constituency home.

She has won plaudits in some quarters for focusing on her high-flying career with law firm DLA Piper rather than being ever present at her husband's side on his tour of key election battlegrounds.

Mr Clegg dismissed suggestions the couple had planned the strategy but in a pointed reference to David Cameron's comments about his wife Samantha he said: "I don't think Miriam regards herself as a secret weapon and I don't either."

He said the Tory plans for marriage tax breaks were a "throwback to the Edwardian era", adding: "Miriam and I got married for love, not for three quid a week. It's patronising drivel."

As she tackled the art of dry stone walling, Ms Gonzalez Durantez said: "I'm here because I want to support Nick. I have always said I will do this when I am not working and I don't have the kids."

The couple's three sons are on holiday with Ms Gonzales Durantez's family in her native Spain.