In an unprecedented show of political unity, the three main candidates in the race to be mayor of London have backed a call for an amnesty for all illegal immigrants who have lived in Britain for four years or longer.

Taking a break from the divisive politics that have characterised their campaigns so far, Mayor Ken Livingstone of the Labour Party, who is seeking an unprecedented third term, made the pledge along with Tory Boris Johnson and Liberal Democrat Brian Paddick.

In making the pledge Wednesday, both Livingstone and Johnson opposed their parties' official position on immigration.

The idea of 'earned regularisation' comes from London Citizens - made up of 2,500 community leaders, faith groups and other organisations - which says similar schemes have been a success in the US, Spain and other countries.

It says undocumented workers should be given the chance to be integrated into mainstream society and obtain papers allowing them to work and pay taxes legally.

The idea also has the backing of Stephen O'Brien, a leading London businessman.

'Enormous untapped potential and appalling waste is taking place right in front us,' O'Brien said Tuesday.

'Business leaders recognise that the case for an amnesty is both principled and pragmatic. The economic and moral case for liberating this army of workers from the underground economy is irrefutable,' he added.

According to some estimates, the number of illegal workers in Britain range from 500,000 to 700,000, half of whom may be failed asylum-seekers. An estimated two-thirds of illegal migrants work in London and southeast England, in cleaning, catering, hospitality and construction. Because they do not have legal rights, their pay and conditions are subject to abrupt changes.

The Greater London Authority estimates the number of irregular migrants in London to be about 380,000.

Livingstone said: 'Migrants contribute hugely to the economic, civic and cultural life of London and the UK. To have a substantial number of them living here without regular status - because of deep-rooted failings in the immigration system, some dating back over a decade - is deeply damaging to London as well as to them.'

'Where people have been here a long time and have no prospect of being able to be sent back, then an amnesty could be considered so that they can pay taxes and legally contribute to the British economy,' said a spokeswoman for Johnson.

Liberal Democrat candidate Paddick said: 'An earned route to citizenship is a solution to one of the greatest challenges facing this country.'