Uber is a "disgrace", UK shadow chancellor John McDonnell said as Labour heavyweights clashed with the Tories over the taxi app firm losing its licence to operate in London.
British Labour leader Jeremy Corbyn said Transport for London (TfL) had raised "serious concerns" about safety but suggested Uber had an opportunity to "mend its ways".
Trade unionists also attacked the firm, with Unite boss Len McCluskey saying it was part of a "horrible, race-to-the-bottom culture" and TUC chief Frances O'Grady calling it a "global bully boy".
But Tory Minister for London Greg Hands said Labour's approach was "astonishing" and claimed the party did not have "the balanced approach the country needs".
Tom Elvidge, general manager for Uber in London, said the company was prepared to meet with officials after TfL announced the decision not to issue it with a new licence on Friday.
Mr McDonnell told ITV's Peston on Sunday: "Hand on heart, I don't think I've ever used Uber.
"The company is a disgrace. You have to abide by the law. If the company was outside the law, what could Transport for London do?
"I think the company is at fault here. Four months ago they were told to get their act together and they didn't."
Mr Corbyn said he was "not sure" if he had ever used an Uber "but I don't think so".
He said Uber needs to offer "decent pay and conditions for its staff" and "there is an issue about safety".
Mr Corbyn told BBC's Andrew Marr Show the dispute was "either going to go to court or they are going to mend their ways and make a new application and TfL will have to reconsider it".
Mr McCluskey told Peston on Sunday he had never taken an Uber and added: "I'm one of these people that believes that Uber is part of this horrible, race-to-the-bottom, culture that has developed in this country."
At a fringe event at Labour's conference in Brighton, TUC general secretary Ms O'Grady praised TfL and London Mayor Sadiq Khan for "standing up to what is a global bully boy".
Mr Hands said: "It's astonishing Labour find it so easy to condemn Uber, a company with 3.5 million users, but so difficult to condemn illegal strike action by trade unions.
"Of course Uber need to meet safety standards, but Labour always take it too far and simply don't have the balanced approach the country needs at this time. Ordinary working people would pay the price for that."
More than 650,000 people have signed a petition calling for the decision not to grant a new licence to be reversed.
Mr Elvidge told the Sunday Times: "We'd like to know what we can do ... to sit down and work together to get this right.
"We haven't been asked for any changes.
"We'd like to know what we can do. But that requires a dialogue we haven't been able to have."
Meanwhile, rival app Lyft is reportedly looking to muscle in on the London market, with a Freedom of Information request revealing the San Francisco-based start-up had held talks with TfL and City Hall in the last year, according to the Sunday Telegraph.