Extensive house building in rural areas and the green belt appeared inevitable last night after the Government ordered the West Midlands to find space for 420,000 new homes by 2026.

The figure, which is 55,600 more than the region's local councils believe is achievable or desirable, puts the Regional Assembly on a collision course with the Department of Communities and Local Government.

A proposal by the assembly to plan for 365,000 new homes by 2026 was this week rejected by Baroness Andrews, a junior minister at the DCLG.

Baroness Andrews said the figure, contained in a draft revision of the West Midlands Spatial Strategy, would not do enough to deliver the Government's agenda to increase housing building across the country.

Prime Minister Gordon Brown has called for a major house building drive across the English regions in order to meet population growth figures and demand for single person accommodation.

The population of the West Midlands is expected to grow by almost one million over the next 24 years, according to a Government report.

Baroness Andrews has ordered the Government Office for the West Midlands to look at options which could deliver higher housing numbers.

The move was criticised by Regional Planning Part-nership chairman Rex Roberts, who warned the Govern-ment's intervention could lead to the "worst of all worlds" in which greenfield sites are snapped up by developers while brownfield land in towns and cities remains undeveloped.

The RPP fears the principle underpinning the spatial strategy - concentrating house building in the major urban areas of Birmingham and the Black Country in order to halt the unsustainable movement of people to the countryside - is now at risk.

Coun Roberts said: "While we recognise the need to improve housing affordability, it is important that we do not fall into the trap of a simple numbers game.

"This could lead to the worst of all worlds if more greenfield sites are released and efforts to regenerate brownfield land and derelict sites is diverted such that the overall urban and rural renaissance agenda is undermined."

Rose Poulter, policy director at the regional assembly, described the Government's housing target as a "moveable feast".

When consultation on the spatial strategy began, the Government was asking for 380,000 new homes in the West Midlands by 2026. That figure has since been increased to 420,000 following advice given to the Government by the National Housing and Planning Advisory Unit, Ms Poulter said.

The WMRA has said a maximum 18,000 new homes a year can be built over the planning period. The Government would like to see more than 20,000 properties constructed each year.

Ms Poulter added: "Assembly members are really concerned about concreting over the green fields and rural areas. They will remain concerned whilst the Government is looking at how this region can increase its housing allocations."

Gerald Kells, regional policy officer at the Campaign to Protect Rural England, said: "The aim of concentrating on urban regeneration and not creating unsustainable new housing estates in the countryside is already at breaking point in the region, bumping up the numbers would mean more badly located greenfield houses and would do little to provide the affordable homes we need."

A WMRA spokeswoman said: "In submitting the draft revision, the assembly made it clear that the policies and proposals put forward represented an ambitious growth strategy for the region but one that was well balanced taking into account a wide range of important considerations.

"The assembly particularly emphasised that it had worked hard with its regional partners to meet the Government's housing objectives and it believed that it had achieved this in a way that maintained the drive towards sustainable development and growth emphasising the urban and rural renaissance so important to the future of this region."