A reunion to mark the 10th anniversary of the collapse of Lehman Brothers has been called "sickening" by the shadow chancellor.
The failure of the US investment bank in 2008 was a crucial moment in the financial crisis.
Mr McDonnell said those who had suffered a "decade of austerity" would be "absolutely disgusted" by the event.
Financial News, which first reported the party plans, said about 200 former staff had been invited.
The trade publication said the gathering planned for 15 September - the date of Lehman's collapse a decade ago - would involve cocktails and canapes.
- How did the 2008 financial crisis affect you?
- 'That was chilling': Ten years since the financial crash
Lehman's failure heralded the start of a global crisis, with governments around the world scrambling to avert a possible financial meltdown.
Its collapse sent shockwaves through financial markets, with then chancellor Alistair Darling saying the bank's collapse "allowed panic to get into the system".
"It set in train events that brought our banks and the American banks within hours of collapse just three weeks later," he said later..
"When Lehman went down, people started circling other banks as well, to look for the next one to go after. The situation got progressively worse."
The bank employed about 5,000 people in the UK and when it went bankrupt, an estimated 25,000 people globally lost their jobs.
An email invite to the Lehman reunion, seen by Financial News, is addressed to "Lehman Brothers & Sisters" and reads: "It's hard to believe it's been 10 years since the last of our Lehman days!... One of the best things about Lehman was the people. What better way to celebrate the tenth anniversary than getting everyone from former MDs to former analysts back together again!"
Mr McDonnell said the party plans were "unacceptable and highly inappropriate".
"This is absolutely sickening after a decade of people suffering austerity. It's particularly disgraceful in the context of all the people who lost their jobs and homes to pay for bailing out these bankers who caused the financial crash," he added.
Financial News said those organising the event were aware it could be seen as controversial, and had tried to keep the details of the event and its location secret.