Proposed new election laws would make it easier for extremist parties to field candidates at the polls and get publicity, say the Conservatives.
BNP leader Nick Griffin polled 9% of votes in Keighley at the election
The Electoral Administration Bill is being debated by MPs on Tuesday. It includes plans to make it harder for candidates to lose their deposits.
Tory spokesman Oliver Heald says the bill will lower the hurdles for extremist parties, such as the BNP.
But the government says it is part of moves to encourage people to vote.
The proposals were welcomed by the BNP. Spokesman Phil Edwardes they would help the party field more candidates in general elections.
But he denied the BNP's policies on "preserving and maintaining traditional British culture" were "extreme".
"Our views are shared by the majority. It is only the media that have told people that to have these views is wrong," he said.
Every candidate at a general election must put up £500 as a deposit.
Under current rules, candidates get the money back if they win 5% of the votes cast in their constituency. The new plans would reduce that threshold to 2% of the votes.
The Conservatives say the measure would have saved the British National Party £36,500 at the last election, allowing it to field candidates in another 73 constituencies.
The Tories also say the move would make it easier for extremist parties to become entitled to have party election broadcasts shown on television.
Parties have to field candidates in at least one-sixth of the seats up for election to have their broadcast shown - something the BNP achieved at the last election.
'Part of democracy'
Tory shadow constitutional affairs secretary Mr Heald said the changes could allow more extremists to use to the free mail shot which is available to each candidate.
Mr Heald said allowing extremists to run for election was a "necessary evil" in a democracy but there had to be checks and balances.
"Lowering the threshold will help the likes of the BNP spread their repugnant propaganda courtesy of the taxpayer, despite the fact that their racist views do not carry public support," he said.
A spokesman for the Department for Constitutional Affairs said lowering the threshold for deposits had been proposed by the Electoral Commission.
The bill was aimed at improving both security for voting and tackling falling turnout at the polls, he said.
A spokesman for the Electoral Commission said: "We believe that candidates should be able to take part in the electoral process without having to meet substantial financial costs at the outset.
"The difficulty with the current deposit system is that it may deter independent candidates and those from smaller parties who have a valuable contribution to make in encouraging participation and interest in the political process."
Other measures in the elections bill include:
- Creating two new offences to prevent electoral fraud, particularly through postal votes
- Holding trials for personal identifiers (such as signatures and dates of birth) to prevent voter impersonation
- Reducing the age at which somebody can stand as a candidate for parliamentary elections from 21 to 18.
The bill has its second reading debate in the House of Commons on Tuesday.