Ed Miliband's threat to break up Britain's biggest banks comes in the wake of the Libor-fixing scandal and further revelations of mis-selling by the high street giants.
Under government plans, banks have until 2019 to ring-fence high street operations handling consumer and small business accounts from high-risk "casino" arms within the same group.
But Labour has said the Coalition are moving too slowly to force change, while accusing Chancellor George Osborne of watering down the reforms called for in the Vickers review in the face of lobbying by the industry.
Mr Miliband said a Labour administration would legislate if major changes were not in place by the 2015 General Election.
The party wants to see evidence of a cultural change and the Vickers report implemented in "letter and spirit", including splitting staffing and corporate governance.