John McDonnell will use his first major speech as shadow chancellor to introduce plans for a 'Robin Hood tax' on stock market trading.
Speaking at his first Labour conference as shadow chancellor, Mr McDonnell will outline his plans for the tax on how it could be used to fund areas of the NHS.
Chancellor George Osborne has previously rejected calls for the tax, claiming it could negatively impact the UK's financial sector.
The Financial Transaction Tax - it's proper name - will be openly debated at the party conference during and following the shadow chancellor's speech. Should there be enough support for the tax, it will become a Labour policy ahead of the next general election in 2020.
Mr McDonnell has long been a campaigner for the Robin Hood tax. He previously fell out with Ed Balls when trying to organise a debate on the issue in the House of Commons.
The shadow chancellor said: "Robin Hood tax at the moment is Labour Party policy on the basis of if we can introduce it globally, and that's been Labour Party policy for some time now.
"However, what we are saying is today we are going to launch a review of our taxation system, we are going to bring the greatest economic minds in the world to bear on that issue, we are going to consult with the British people and then we will arrive at a decision on the way forward."
Despite Mr Osborne's warnings, the Robin Hood Tax campaign group says there is nothing "dangerous or radical" about the idea.
David Hillman, from the group, said the point of the tax is "to have the banks and hedge funds pay for, or at least contribute to paying for, the immense economic damage their gambling, essentially it was gambling, had caused".
John McDonnell has warned that his speech could be a little boring, ruling out calls for big tax increases. He will say that a Labour government would live within its means, invest in the economy and spread wealth "more equally".
So, if not through tax increases as suggested by national media and the Conservatives, how does Labour plan to raise funds to reduce spending cuts? One of the answers is tax avoidance.
Mr McDonnell will use his speech to announce a review of HMRC, to investigate how it could do more to collect the billions of pounds in tax that businesses and individuals avoid paying.