Ray Ruddick, who drives a battered Transit van, is officially listed as having contributed more than £104,000 to the national party's coffers since Mr Brown became Prime Minister less than five months ago.
His contributions, combined with those of a woman he is linked to, make him Labour's third biggest donor under Mr Brown, behind Lord Sainsbury who gave £2million and Anglo-Iranian businessman Mahmoud Khayami, whose latest donation in September was £320,000.
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Jobbing builder: But Ray Ruddick is the Labour party's third main backer
Since 2003, Mr Ruddick's total contributions to Labour are listed as £196,000. But standing outside the semi he bought for just £12,000 and wearing a paint-splattered fleece top, the 55-year-old initially told The Mail on Sunday he had no recollection of ever giving to the party.
However, Mr Ruddick works closely with David Abrahams, a wealthy property developer who is a prominent figure in the North East Labour Party.
He was in the front row when Tony Blair made a speech to activists in his Sedgefield constituency in May announcing his decision to quit as Prime Minister. Last night, 53-year-old Mr Abrahams, the son of a former Lord Mayor of Newcastle, refused to say whether it was, in fact, him who funded the donations.
The Labour Party refused to discuss what checks they made before accepting Mr Ruddick's donations, saying: "We are totally satisfied that these donations are in line with the rules."
But the Tories called on Labour to give a full explanation as to how a builder with modest means and no interest in politics could come to be one of the party's leading sponsors.
Conservative frontbencher Chris Grayling said: "There is clearly something extremely unusual about all of this. Given the fact that Gordon Brown promised to bring a fresh approach to British politics, he and the Labour Party need to give a pretty clear explanation of the circumstances surrounding these donations."
The disclosures follow a Mail on Sunday investigation into the latest list of Labour donors, published on Tuesday. It shows that in the last financial quarter Mr Ruddick and a woman listed as Janet Kidd both gave £80,000 on the same day in July.
Nor is this the first time that the pair have donated, having contributed a grand total of £381,850 in the past four years, including a combined £62,000 just two days after Mr Brown became Prime Minister on June 27.
Janet Kidd: Gave an £80,000 donation to Labour in July
Mrs Kidd, who is believed to work as a secretary for Mr Abrahams, is also listed as a £5,000 donor to Harriet Harman's successful Labour deputy leadership bid this year.
When first approached by The Mail on Sunday at his home on Thursday and told that political donations had been made in his name, Mr Ruddick said: "Has there? I'm not making any comment."
When we returned yesterday he again initially declined to talk, before driving to Mr Abrahams's house. Then, after explaining he was scared of losing his job, he said: "I can honestly tell you that under no circumstances have I made any donations to the Labour Party."
He said neither he nor Mrs Kidd had heard of the donations until The Mail on Sunday started making enquiries. " There's quite a possibility that Janet and myself have been set up," he said.
Asked if he knew David Abrahams, he said: "I'm not saying either-or. I cannot go into this any more. I do not know the ins and outs of it."
Asked about the donations in his name totalling £196,000, he said: "You obviously know that with me coming from a council house, there's no way that's possible. You're barking up the wrong tree."
Neither he nor Mrs Kidd were interested in politics and neither could afford to donate vast sums, he said, adding: "I've got my mortgage to pay. I've never voted in my life. I can't stand Labour. I can't stand any politicians."
He said Mrs Kidd worked as secretary to Mr Abrahams, adding: "Her husband's not happy with her. We work together. She's under a lot of stress."
Mr Ruddick's Sixties-built three-bedroom semi stands on a busy dual carriageway in the working-class Blakelaw district of Newcastle. Land Registry records show that he bought it for £12,000 seven years ago.
Friends call him a "jobbing builder", but Companies House records list him as a director of four property companies where Mr Abrahams is the co-director.
In some cases Mr Ruddick, Mrs Kidd and Mr Abrahams are listed as sole directors of the same firm.
Mrs Kidd, 56, refused to come to the door of the £170,000 semi-detached home she shares with her husband, Michael, in Whickham, a modest suburb of Gateshead, to discuss the donations.
Mr Abrahams, who made a £5,000 political donation in his own name to Hilary Benn's failed deputy leadership bid, is no stranger to controversy.
He was selected as Labour's candidate for William Hague's seat of Richmond in North Yorkshire for the 1992 General Election.
However, the local party ousted him months before the Election following claims that he had misled members about his marital status.
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Powerful friends: David Abrahams, circled, at Tony Blair's farewell speech in Sedgefield
According to newspaper reports at the time, Mr Abrahams attended his first selection interview with a woman and a child who were said to be his family – although he was not married. At the time he said he had been a victim of a "smear campaign".
The same year Mr Abrahams, a former Tyne and Wear councillor and son of one-time Lord Mayor of Newcastle Bennie Abrahams, faced a Crown Court trial for illegally evicting a tenant from a flat he owned. He was cleared of all charges.
Mr Abrahams is listed in Companies House as a director of six companies, all based in Newcastle. These are: Park View Property Investments, Acorn Residential Estates (North Tyne), Acorn Residential Estates (Durham), Acorn Commercial Developments (Northumbria), Foxlow and Webfast. Mr Ruddick is also either a director or secretary of five out of six of these companies. Similarly, Mrs Kidd holds an official position in most of Mr Abrahams's firms.
Mr Abrahams runs his businesses from the old family home in the upmarket suburb of Gosforth, but neighbours say they rarely see him there. He has knocked two substantial semi-detached houses into one and added extensions.
The combined property would be worth about £900,000.
The address Mr Abrahams gives at Companies House is a flat in a mansion block in fashionable St John's Wood, North London.
The Mail on Sunday yesterday put a number of questions to a lawyer who contacted us on behalf of Mr Abrahams.
We asked if Mr Abrahams had contributed to the Labour Party and we requested details of the dates and amounts.
We also asked if Mr Abrahams had provided the money for the donations made by Mr Ruddick and Mrs Kidd.
The lawyer stated that Mr Abrahams was not prepared to reply because he had not been given sufficient time to do so.
Last night – eight hours after initially saying he did not know about the donations – Mr Ruddick gave a different version of events. He said that he and Mrs Kidd had, in fact, made the most recent payments to the Labour Party.
Mr Ruddick said: "I made donations to Labour on that day which totalled £80,000. Janet also made the same size donations on the same day. These are the first payments either I or Janet have ever given to Labour."
But, strangely, he also repeated his statement: "I do not like the Labour Party or politicians."
He would not say why they had made the donations. And he said other donations made in their names in recent years had been paid for by other individuals, whom he declined to name.
A spokeswoman for the Electoral Commission watchdog said: "Under the Election Act 2000 a person giving money to a party on behalf of someone else must inform the party they are doing so.
"In turn, the party must provide that information to the Electoral Commission. This is in the interests of ensuring transparency in funding of political parties."
The Labour spokesman added: "The Labour Party upholds the highest standards. We have been receiving donations from these individuals for some time.
"These donations have been declared in full to the Electoral Commission and are from permissible donors. We are totally satisfied these donations are in line with the rules."