White candidates should be barred from standing for Parliament in up to eight constituencies in order to get more black and Asian MPs elected, says a controversial report commissioned by Labour's deputy leader, Harriet Harman.

Positive discrimination is illegal in the UK, but the report concludes that, without a change in the law allowing parties to impose all-black shortlists, it would take more than 75 years for Britain's ethnic make-up to be fairly reflected at Westminster.

The findings come amid questions over Britain's failure to develop a home-grown equivalent of Barack Obama, the black presidential hopeful. Although there have been three black cabinet ministers since 1997 - Paul Boateng and Valerie Amos, both of whom have since moved on, and Baroness Scotland, the Attorney General - no politician of the stature of Obama, US Secretary of State Condoleezza Rice or her predecessor, Colin Powell, has emerged.

Harman is understood to be still considering the report's findings in detail, but has expressed personal support for a change.

Simon Woolley of Operation Black Vote and the author of the review, said talented candidates were not 'getting past go' at the moment. 'The change in the law is not a sledgehammer to crack a nut; it's not forcing parties to use all-black shortlists,' he said. 'But unless we take positive action measures we are not going to have a representative democracy for more than 75 years. It's not that we don't have Obamas, but we don't have the mechanisms for them to see the light of day.'

His report is understood to conclude that all-black shortlists would be needed for two decades, after which talented candidates could be expected to make it on their own. It identifies 100 constituencies with large ethnic minority communities as prime targets for shortlists, but concludes that positive discrimination would be needed in only four to eight of those seats for four elections in a row to ensure that the proportion of ethnic minority MPs matches the proportion in the population.

Woolley's findings are likely to be controversial, with any proposal to change the law risking a rough ride in the Commons. Last week, former minister Keith Vaz introduced a backbench bill proposing all-black shortlists, which was instantly condemned by Tory backbencher Philip Davies as 'politically correct' and divisive.

However, Vaz is lobbying Harman for the measure to be included in a bill on equality issues later this year - meaning it could be on the statute book by 2009. 'She is the person who has a huge history of supporting these issues,' he added.

Woolley said that a change in the law could help the Tories by putting David Cameron's A-List - a pool of candidates, many of them women or from ethnic minorities, singled out for the fast track to the irritation of some white men - on a firm legal footing.

His report was commissioned by Harman last autumn to examine the merits of positive discrimination. Only 2 per cent of MPs are black or Asian, compared with more than 7 per cent of the general population.