Anti-BNP campaigners say the party is still a long way from being a credible force in spite of a record £200,000 donation.
Hope Not Hate said Nick Griffin's party was increasingly reliant on bequests. The BNP said some people preferred to leave money in their will because they feared a "leftist" backlash.
The far-right outfit was handed £200,000 in a two-part bequest from Albert Edward Stanmore, believed to be a former tube driver who was born in London.
The second £100,000 tranche, from the final quarter of 2012, is revealed in the latest batch of party donations published by the Electoral Commission.
BNP spokesman Simon Darby told The Huffington Post UK that the total donation was "probably" the largest in the party's history.
He said: "Our people are very committed, they don't just do this as a hobby."
Stanmore had been a long-term BNP member, Darby added.
Dan Hodges, a spokesman for Hope Not Hate, which campaigns against the BNP, said the BNP was still "massively crippled by debt" dating back to previous election campaigns.
He added: "This obviously will help, but it is a drop in the ocean and they are still a long way from being a viable political party."