Tough new ethical guidelines for the mayor of London will strip Ken Livingstone of his£120,000 annual media earnings if he wins the election on 4 May.

Under the anti-sleaze code drawn up by the Government, the former GLC leader will be forced to quit all of his directorships and have his conduct monitored closely by an ethics committee, The Independent has learnt.

The mayor's power of patronage will also be severely curtailed. An "independent" figure, with no links to the mayor, will have to be consulted on all appointments.

The ethical guidance, which was placed in the House of Commons library by John Prescott yesterday, makes clear that the role of the assembly will be "to examine and test" the mayor's policies and strategies. As a result, the "mayoral handcuffs" would in effect shackle Mr Livingstone and give the Greater London Assembly the power to rein in any policy it deems unsuitable.

With the 25-member assembly likely to be dominated by the Labour Party and led by its deputy mayoral contender, Trevor Phillips, the rules mean Mr Livingstone will be given little room for manoeuvre.

The guidance reflects Downing Street fears that if Mr Livingstone wins the mayoralty, as the polls currently predict, then he should be tightly bound by the assembly.

With £3.3bn at the mayor's disposal, ministers are keen to ensure that public money is not wasted and have already insisted that the assembly can block his budget with a two-thirds majority. A special standards committee, set up by the assembly, will also be appointed to "promote and maintain high standards of conduct" by the mayor, with wide-ranging powers to discipline him.

However, it is the explicit ban on the mayor and deputy mayor from holding any company directorships that will particularly hit Mr Livingstone. The ban, which does not apply to assembly members, is a much tougher restriction than is imposed on MPs at Westminster.

The Brent East MP is the director of Localaction, a company that channels all of hisearnings from media work and after-dinner speeches. Mr Livingstone was forced to apologise to the Commons earlier this year for failing to declare £158,000 in outside earnings.

The mayor's power of patronage allows him to appoint a personal cabinet of 10 people. He chooses who chairs and sits on the Transport for London authority, the Metropolitan Police Authority and the London Development Agency. But Mr Prescott makes clear an "independent element" will be expected to oversee the process.

In a separate move, the Tory party launched a "Get Ken" campaign yesterday in a last-ditch attempt to persuade Conservative voters not to vote for Mr Livingstone as mayor.