Boris Johnson and the London borough of Southwark are at loggerheads over the issue of affordable housing.

A spokesman for the London mayor said is ‘absolutely committed to maximising affordable housing delivery' after Southwark LBC said it was considering legal action over plans to allow developers at sell affordable housing at 80% of the market value.

‘Rents average 65% across the programme, and all rents within housing benefit caps so that those who are reliant on housing benefit are able to claim it,' said the mayor's spokesman.

‘The London Plan conforms with national planning policy. Rent controls across 33 different boroughs would simply have the negative effect of driving away vital investment and reduce the number of homes built – exacerbating London's housing crisis.'

Southwark currently sets social rent at around 40% of the market value, but according to rules set out in the mayor's London Plan, developers will be able to charge 80% of the market rent and still call them affordable, in order to meet quotas set out in planning applications.

‘We are very keen to seek a judicial review of this decision,' said Southwark's cabinet member for regeneration and planning, Cllr Fiona Colley. ‘Councillors need every power possible to ensure rent levels are appropriate and affordable for their residents.

‘Maybe there are some areas of London where rent levels of 80% of market rent are affordable to most people, but they certainly aren't in Southwark.'