"What happened was with the leak of the Panama Papers, we discovered just how the integral links between companies and the City of London and tax evasion, and that's unacceptable," he said.
"So what we want to do is make sure there's proper enforcement and actually make sure we have a proper legislative base. What's interesting in the City now is that's what a lot of people are saying as well. The good do not want to be brought down by the bad," he said.
Although McDonnell also criticized some large companies for exploiting their staff.
"There's been shocking examples of abusive practices in some industries and some sectors and what we want to do now is introduce company law that makes sure employees are treated fairly and properly. We don't want those examples ever again. We had one example of one woman who was terrified of taking time off that she gave birth in a toilet, that's not acceptable."
"What we want to do is make sure we have an economy that's prosperous, but it's got to be fair as well," he added.
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