Economic growth in the Black Country over the next 25 years will trigger significant expansion in retail provision with the construction of four huge shopping centres, each one the size of Birmingham's Bullring.

The forecast is contained in a development strategy drawn up by the Regional Planning Body.

RPB members suggest in their Black Country Study that consumer spending on shopping in Wolverhampton, Walsall, West Bromwich and Merry Hill will rise by 300 per cent by 2031, leading to demand for a 400,000 square metre increase in shopping floorpsace.

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However, the figures are based on assumptions that the Black Country population will rise sharply and that income levels will increase to national average levels.

Both scenarios may be over-optimistic, according to Birmingham City Council, which has warned that too much retail growth in the Black Country could harm existing shopping centres elsewhere in the West Midlands.

Setting out its vision, the Black Country Study document says: "In 2033, the Black Country is a confident 'we can do it' place. We are proud of our skills, our proficiency and out work ethic - these are key to our prosperity.

"The Black Country is now a truly polycentric city made up of four thriving and distinct retail commercial city centres, each one playing a unique role."

Responding to a consultation exercise on the Black Country Study, David Pywell, Birmingham's strategic director of development, said there would be risks if planners committed to almost half a million square metres of shopping floorspace but growth in incomes and spending power did not materialise.

"The out-come would be a substantial increase in the market share of the Black Country centres, which would be at the expense of centres elsewhere," Mr Pywell said.

Birmingham is urging the RPB to curtail growth in Black Country shopping centres at a level which maintains existing market share.

The city council is also concerned about forecasts of a "very large decline" in the manufacturing sector and rapid growth in office-based occupations contained in the first phase of the Black Country Study.

Alternative forecasts showed the decline in manufacturing might not be so pronounced, Mr Pywell added.

The study suggests an increase in the Black Country population over 2001 levels of 60,000 people by 2021 and 125,000 people by 2031.