Malcolm Scott, who according to a judge had gone missing earlier this year, appeared at the Tories' summer party in London along with the Prime Minister and his Cabinet.
Labour MP Graeme Morrice said the revelation was a disgrace.
Scott was until recently one of the party's biggest donors north of the Border, pouring in more than £1.6 million in corporate and personal contributions.
He was close to Foreign Secretary William Hague, allowed David Cameron use of his private jet, and even hired singer Bryan Ferry to play at his birthday party.
However, the grain and property tycoon's empire was put into administration last year after his companies failed to repay millions of pounds in banks loans.
Scott was then sequestrated, the Scots term for bankruptcy.
His mother, father and wife were also made bankrupt as a result of the financial collapse, although it is believed his mother's sequestration has since been discharged.
One of Scott's creditors, Courseworks Ltd, also went to court over nearly £740,000 in unpaid gardening and landscaping work on Scott's former mansion in Kirknewton.
The jobs included £15,548 for a tennis court wall, £16,574 for a boathouse feasibility study, £21,415 for a walled garden greenhouse base, £3290 on a "lake survey" and £1500 on bark.
Administrators working on Scott's case have also been trying to trace assets such as £215,000 of Jordanian artefacts and Persian rugs worth nearly £45,000
At the Courseworks hearing in February, Judge Lord Drummond Young said it seemed Scott had disappeared and was trying to avoid contact with Coursework's legal representatives.
However, despite his apparent penury, Scott was spotted at the UK Conservatives' summer party on June 24. The gala event was held at the Old Billingsgate Market and tickets were believed to be selling for between £1000 and £1800.
Auction prizes at the fundraiser reportedly included grouse shoots, a portrait of Margaret Thatcher and a bust of Cameron, which sold for about £90,000.
It was reported that the menu for the occasion included smoked rainbow trout, guinea fowl stuffed with herbs and mascarpone, and rhubarb and elderflower tart with gingerbread cream.
The A-List crowd included London mayor Boris Johnson, Cameron and most of his Cabinet.
The event was sponsored by Shore Capital, an investment firm run by Tory-supporting stockbroker Howard Shore, who once said: "I would say that anyone employing fewer than 10-20 people should be free from employment legislation, save for discrimination."
Although a judge said in February that Scott appeared to be missing, it is understood he represented himself at another sequestration hearing recently.
One of the creditors involved in the case applied for Scott's sequestration to be extended.
Morrice, the MP for Livingston, said: "It is a disgrace that a bankrupt like Malcolm Scott, who has left his creditors out of pocket, attended a summer party alongside the Prime Minister.
"It is also a slap in the face for the individuals and businesses who are still waiting for their money, to see Scott at such a glitzy and expensive event."
An SNP spokesperson said: "The Tories have been keen to wash their hands of this situation, but Mr Scott's attendance at this high-powered party event means that is no longer possible. The longer they remain silent on this matter, the worse it looks for them.
"The creditors who have lost so much are unlikely to look kindly upon Mr Scott's being welcome at this event and at the very least the Tories owe them a public apology."
A Tory spokesman did not respond to a request for comment.
Scott could not be contacted.