Shadow chancellor John McDonnell has attempted to distance himself from demonstrators who flew Syrian regime and communist flags while he spoke at protest.
The senior Labour figure said he was unaware, and that he felt “furious” after learning about the banners flown in Trafalgar Square for the annual May Day rally.
He was criticised by British Ukrainians for speaking under the Communist hammer and sickle banner at the event.
Photos from the scene also show some people at the mass rally holding portraits of Joseph Stalin, and other Soviet iconography.
The Association of Ukrainians in Great Britain (AUGB), reacting earlier this week, said politicians had "no excuse" for sharing a platform with others carrying banners of Stalin - a "tyrant" who killed millions of their countrymen, and a hammer and sickle representing "repression and mass murder".
Mr McDonnell, asked if he knew he was standing in front of the controversial banners, told BBC Radio 4's Today: "No, of course I didn't know.
"It was a TUC march, it was a TUC May Day celebration. I was there to celebrate May Day, which was a celebration of the contribution workers make to our economy."
Asked if he would have insisted on the banners being removed if he had known about them, Mr McDonnell said: "Of course I would, my goodness yeah.
"It was only afterwards - the TUC organisers didn't know they were there, someone had obviously shoved them up, I was furious afterwards and I think so were the organisers themselves.
"We're talking about the Trades Union Congress. It isn't an organisation that backs any of these sort of movements. That's appalling."
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