The BNP is likely to improve its share of the vote in some key regions by 50% at the next general election, a leaked Whitehall report has forecast.

A study of the growing influence of the far-right party found that 18% of those polled in three areas where it is strongest said they would vote BNP.The report says that would be an increase from 12% in previous polls.

The study, for the Equality and Human Rights Commission, the race watchdog, is the most comprehensive official analysis of the BNP produced. Supported by polling from ICM, it was based on three samples of 1,500 people in Blackburn, northwest Leicestershire and Stoke-on-Trent.

The three areas were chosen because they are among those where the BNP has developed some of its strongest support.

This weekend Phil Woolas, the borders and immigration minister, said the analysis showed the BNP now posed “a significant threat” to the main parties.

The poll is supported by focus group research highlighting fears about the impact of immigration on jobs, schools and other public services. The BNP’s policy is to stop all “non-white” immigration. It proposes “firm but voluntary incentives for immigrants and their descendants to return home”.

The disclosure that almost one in five voters in some areas say they plan to back the party will set alarm bells ringing in government. Michael Thrasher, professor of politics at Plymouth University, said 18% was not enough to win the BNP seats in parliament. However, it could split the Labour vote in some seats, such as Blackburn, where Jack Straw, the justice secretary, is the MP. The Tories came second there in the 2005 election.

Woolas, MP for Oldham East & Saddleworth, said he was surprised at the high level of BNP support in Blackburn, but that in Stoke separatist Islamic groups were creating a “very worrying” backlash of support for the BNP. He said: “The British National party is a significant threat in electoral terms in parts of our country as this poll confirms. The mainstream parties must focus on grassroots politics. This can’t be done in television studios in London. It has to be done in the streets.”

Details of the report are being kept under wraps pending publication in the next few weeks. But it will reinforce concerns the BNP is growing.

At the 2005 election the BNP obtained 0.7% of the vote. In June this year it won its first two seats in the European parliament, in Yorkshire and the Humber, and the northwest, where Nick Griffin, the party chairman, won 8%.

Times Recommends

• Please help: maimed children of war want their lives back
• Unlocking the cruellest prison
• Prince Charles named in £81m court battle