A major British family investor, the first to be named by the businesswoman Amanda Staveley as a financial backer of her £250m bid to buy Newcastle United, has said they are not now planning to invest in the club. According to both parties, the Reuben family did agree financial terms to support Staveley’s bid in December, but after her offer was rejected the family has not been involved with it since.

A representative of Staveley, asked by the Guardian how she is proposing to fund the planned takeover of the club from the Sports Direct owner Mike Ashley, said on Wednesday that he had been authorised to reveal that the London-based Reuben family is one of her co-investors. Before that, Staveley had not publicly named any backers; sources close to her had said she has £28bn under management from investors worldwide but that she might mostly pay for Newcastle herself.

Organised corporately as Reuben Brothers – a property, private equity and venture capital conglomerate headed by the brothers David and Simon Reuben, who are reported to be billionaires – the family have a very sizeable and financially powerful business. The brothers own Millbank Tower and a very large property portfolio in London, the value of which escalated them to the top of the Sunday Times “rich list” in 2016 with an estimated personal wealth of £13bn. They are also involved in plans to convert a former police station in Newcastle city centre into a “retail and leisure quarter” featuring a hotel, shops and restaurants.

Staveley’s representative said that the Reuben family’s proposed investment in Newcastle had been led by David’s son Jamie. He is reported to have various ventures in his own name and to be a football supporter, and has an office at the family’s charitable arm, the Reuben Foundation.

A source close to Jamie Reuben confirmed that they were approached by Staveley in November, and they did agree financial terms for their involvement. Then in December they understood that the deal was off because Staveley’s offer had been rejected, and the family has had no further engagement with it since then.

The official spokesman for Reuben Brothers confirmed that the family did consider whether they might invest in Newcastle, but said Simon Reuben is unequivocal that they are not doing so. “It is true that we did have a look at it, but nothing actually happened,” the spokesman said. “We are not investing in Newcastle United either as Reuben Brothers or as any individuals, including Jamie.”

The Reuben Brothers’ official spokesman said Simon Reuben was “pretty emphatic” that the family is not investing and said that, as Jamie Reuben had led on the talks with Staveley, Simon had called Jamie and confirmed that was correct. “They had a look apparently, but it didn’t go anywhere,” the spokesman said.

On Tuesday Ashley, who wants £350m for the club he has owned for approaching 11 years, broke his silence of three months since putting the club up for sale, by denying that there is any deal “on the table” from Staveley, or that discussions are still being held with her. Staveley and her camp say they made the revised offer of £250m in December via a London lawyer, and have been waiting for a month for Ashley to come back to them.

Staveley’s representative said there was an agreement with the Reuben family, and there had been no need to engage further with them since December, or for them to commit any funds, because no further progress had been made with Ashley.