Children's Secretary Ed Balls offered only lukewarm support for the embattled Work and Pensions Secretary, saying that he was "getting on with his job".

Meanwhile shadow chancellor George Osborne was drawn deeper into the donations row as an official complaint was lodged over his failure to declare £500,000 given to support his office running costs in the Register of Members' Interests.

Mr Balls said it was essential that Mr Hain now answered all the questions he was facing in the two inquiries currently under way into his failure to declare £103,000 in campaign donations.

"Of course it is important that Peter answers all the questions in the inquiry, but at the same time he is getting on with his job," Mr Balls told BBC Radio 4's The World at One.

"Of course we have to have the highest standards in public life. We have to answer the questions, we have the kind of scrutiny which you should have, but at the same time I think it is important that we get on with the job of delivering for the British people."

The comments by Mr Balls - one of Mr Brown's closest Cabinet allies - are likely to be seen at Westminster as a further sign that support for Mr Hain is slipping away.

The Work and Pensions Secretary arrived alone for Tuesday morning's meeting of the Cabinet, although he later left No 10 with Environment Secretary Hilary Benn and Northern Ireland Secretary Shaun Woodward.

The Electoral Commission - the official elections watchdog - is currently investigating Mr Hain's failure to inform it of the donations under party funding laws.

At the same time, the Parliamentary Standards Commissioner, John Lyon, has launched an inquiry into whether he broke House of Commons rules by failing to declare the donations in the Register of Members' Interests. The Electoral Commission has not ruled out the possibility that the matter could be referred to the police if it concludes that the law has been broken.