The BNP "seriously overstretched itself financially" in 2005, chairman Nick Griffin said in its annual accounts.
The BNP is led by Nick Griffin
The cash crunch was caused by a dip in donations coupled with the cost of fighting a record 118 seats at the general election, said Mr Griffin.
And although cashflow had improved, "efforts to permanently increase income or trim expenditure are clearly needed," he added.
BNP membership went up by just 146 to 6,502 in 2005, the accounts reveal.
This is despite Mr Griffin claiming a "fresh wave" of members joining after the 7 July attacks on London - which he said "startlingly vindicated" its views.
The British National Party's accounts - published on the Electoral Commission's website - reveal annual losses of £94, 711 in the year to December 2005, up from £20,233 in 2004.
The party's overall debts stand at £52,512.
The cash shortage "seriously hampered" the party's growth in the second half of 2005, said Mr Griffin.
The accounts do not include this year's local elections, which saw the party double its number of councillors.
They show the party raised £672,246 in 2005, mainly from membership fees and the sale of party literature and lottery tickets. That was down from a record total of £718,099 in 2004, when it fought European elections.
The BNP did not receive any donations from businesses in 2005 - but Mr Griffin said its sixth annual Red, White and Blue Family Festival saw a "good number of local commercial traders on site".
And this - together with the absence of far left demonstrators, a "big increase the number of people bringing families with children" and a reduced police presence at the event - were "welcome signs of further normalisation", he added.
Among other small parties, George Galloway's Respect coalition made a surplus of £35,256 and had 5,674 members, up from 3,751 in 2004.
The Green Party made losses of £6,240 but had £23,548 in the bank, thanks to its Greenquest fund, which gets income from donations and interest on money not loaned to the party, but kept in a special bank account. It has 7,110 members - up from 6,281 in 2004.
The UK Independence Party has yet to file its accounts but said in a statement it has no debts.