The Times has learnt that Mr Blunkett spent nearly £100,000 on the dispute with Kimberly Quinn to establish that he was the father of her son, William, and to secure access rights.

When he bought the £15,000 shares in DNA Bioscience on April 21, Mr Blunkett knew that he would have to sell them or put them into a trust, if, as he expected, he was brought back to the Cabinet.

On May 6, the day after the election when Mr Blunkett was made Work and Pensions Secretary, he transferred them into a trust created for his sons Andrew, Alastair and Hugh.

Before embarking on the hugely expensive legal process with Mrs Quinn last year. Mr Blunkett, then Home Secretary, sought the advice and support of his sons.

They met at his Yorkshire home over a weekend in late summer last year after his relationship with Mrs Quinn had become public. It was an emotional family conference.

Mr Blunkett is close to his sons, whom he sees most weekends and often speaks to on the telephone. They all backed his wish to secure legal confirmation that William, the two-year-old son of Mrs Quinn, was his child. Without it he could not have access rights to him.

One friend of Mr Blunkett said: “They were supportive of his wish to establish legally that he was William’s father. There is a filial link. They knew that William was their brother.”

When the relationship broke down in August last year as the affair became public, Mrs Quinn hoped that by pursuing a legal option Mr Blunkett might abandon his wish to have access to the boy. But Mr Blunkett instructed Bindman & Partners, favoured solicitors of the Labour Party, to act on his behalf. Catherine Gieve, a partner in Bindman, is married to Sir John Gieve, who was the Permanent Secretary at the Home Office at the time. Julia Thackray, a partner, acted for Mr Blunkett. Peter Jackson, a QC, was instructed to work on the case.

Bindman partners charge about £250 an hour. The costs were raised by a series of hearings in the High Court including one on the day of the Queen’s Speech.

A second DNA test confirmed that Mr Blunkett was William’s father. Mr Blunkett also sought legal redress to gain access to his son.

At the family summit the sons also backed Mr Blunkett’s wish to pursue a second paternity case to establish who was the father of Mrs Quinn’s then unborn child. He was convinced he was the father. The friend added: “Huge costs were incurred. I never thought for a minute that money would have been a factor in deterring Blunkett from establishing the parentage. His sons were supportive because they have such a tremendous relationship with their dad.

“Blunkett is not rich. His family had no money. This would have been a huge strain on his finances. There were half a dozen court cases. He would want to do right by his sons.”

The DNA test on Mrs Quinn’s second son, who was born in February, showed that Mr Blunkett was not the father.

Times Recommends

• Commons Speaker isolated over leaks row
• Promotion for PC who ignored desperate victim
• Britney’s X Factor comeback angers fans