Mr Gauke, who represents South West Herts, was a non executive director at internet bank Ivobank.

However, he claims he gave up the post in April for 'commercial reasons' and before the new rules were announced.

The shadow treasury minister hasADVERTISEMENTrefused to say how much he earned from the job, which took up 10 days a year.

"I am not disclosing that," he said.

"It didn't take up an awful amount of my time. I agreed to cease to be a non executive director in April before the rules were announced.

"It was for commercial reasons. They were restructuring the board and in a run up to a general election I wanted to cease my outside interest.

"It's nothing to do with not wanting to disclose the information. It was a question of giving up the outside interest in any event for other reasons."

Mr Gauke defended second jobs, saying it gave MPs an 'understanding of the outside world'.

As part of efforts to improve transparency the House of Commons has introduced rules which mean from July those with outside interests must declare how much they earn from them.

Tory leader David Cameron has announced that the shadow cabinet must give up their lucrative second jobs by the end of the year in preparation for a general election.