But while rents are high, the quality of many privately let homes is low, politicians say.

According to the London Assembly, an elected body which champions issues of concern to Londoners, one-third of private rented accommodation is below levels considered acceptable in the social rented sector, and one-in-three private landlords is a 'rogue operator'.

Poor sanitary conditions, faulty kitchen facilities, dangerous boilers and noise are among the problems private tenants experience.

To raise standards, the London Assembly wants the London mayor, local authorities and central government to implement proposals contained in its report, 'Bleak Houses'. These include introducing an 'accreditation badge' for landlords who provide an acceptable minimum standard of accommodation.

The assembly wants lengths of tenancy extended from six months or a year, to up to five years, and better protection for tenants against 'retaliatory eviction' - a situation in which a landlord evicts a tenant for complaining about living conditions.

It also wants the government to consider offering landlords tax incentives to improve their properties and provide longer tenancies.

Jenny Jones, chairwoman of the London Assembly's planning and housing committee, which published the report, said it highlighted the 'awful' conditions some Londoners were being forced to endure.

'Every month of delay in implementing the recommendations will see more people suffering bad conditions and being evicted for complaining about them,' she said.

Some local authorities are taking action against 'rogue' landlords.

Newham council in east London, where the London Olympics will be held and where many Chinese and Hong Kong investors have bought rental properties, is running a pilot scheme to license landlords. Only those deemed responsible are given a licence and those who rent out properties without it are prosecuted.

The London Assembly wants the London mayor to consider extending this scheme across the city.

Candidates in next May's London mayoral election want the letting market reformed.

Incumbent Conservative mayor Boris Johnson supports the introduction of a landlords accreditation scheme.

Labour candidate Ken Livingstone wants rent controls introduced, under which a tenant should pay no more than one-third of their salary in rent. This could result in landlords' rental income being cut.

He also wants a non-profit, London-wide lettings agency set up to put 'good tenants in touch with good landlords'.

Theresa Wallace, head of southeast lettings at estate agency Savills, considers rent controls unlikely, because they would lead to a contraction in the private rental sector.

'[Ken Livingstone] will have to get something through parliament and he will struggle to do that,' she said. 'It would cause maximum problems for landlords who would come out of the market.'

Wallace was not convinced an accreditation system for landlords would work, because schemes operated elsewhere in the country were 'weak'.

Tim Hyatt, president of the Association of Residential Letting Agents, said a client money protection scheme run by his association could be extended if regulation became mandatory. This meant that if an agent failed, the landlords could get their money back.

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Tenancy deals of this duration, in years, are being advocated by the London Assembly, which also wants an 'accreditation badge' for landlords