A reunion on the occasion of the tenth anniversary of the collapse of Lehman Brothers was called "nauseated" by the shadow chancellor.
The failure of the US investment bank in 2008 was a crucial moment in the financial crisis.
Mr. McDonnell said that those who had undergone a "decade of austerity" were "absolutely disgusted" by the event.
Financial news, which for the first time reported the party's plans, said that around 200 ex-employees had been invited.
The trade publication said that the meeting scheduled for September 15 – the date of Lehman's collapse, a decade ago – would consist of cocktails and canapés.
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- & # 39; That was chilly & # 39 ;: ten years after the financial crash
The failure of Lehman heralded the start of a global crisis, where governments around the world tried their best to avert a possible financial collapse.
The collapse of the bank caused shockwaves in the financial markets, and then Chancellor Alistair Darling said that the collapse of the bank allowed "panic to enter the system."
"It triggered train events that our banks and US banks received within three hours of collapse," he said later.
"When Lehman went down, people began to circle other banks, to search for the next one that went on, and the situation got worse."
The bank had around 5,000 people in the UK and when it went bankrupt, an estimated 25,000 people lost their jobs worldwide.
& # 39; Very inappropriate & # 39;
An e-mail invitation for the Lehman reunion, seen by Financial News, is addressed to "Lehman Brothers & Sisters" and reads: "It is hard to believe that it has been 10 years since the last of our Lehman days! … One of the best things about Lehman were the people, what better way to celebrate the tenth anniversary than getting everyone back from former MDs to former analysts! "
Mr McDonnell said the party plans were "unacceptable and highly inappropriate".
"This is absolutely pathogenic after a decade of cutbacks, and it is especially shameful in the context of all those who have lost their jobs and homes to pay for saving these bankers who caused the financial crash," he added. toe.
Financial news said that those who organized the event were aware that it could be considered controversial and that they were trying to keep the details of the event and location secret.