Nick Griffin is complaining he had faced a 'lynch mob'
Peter Hain says his fears have been proved right after a poll suggested support for the BNP has risen after Nick Griffin appeared on Question Time.
A YouGov poll in the Daily Telegraph suggests 22% of people questioned would "seriously consider" voting BNP.
The Welsh secretary said: "The BBC has handed the BNP the gift of the century on a plate and now we see the consequences. I'm very angry."
The show was watched by a record eight million people on Thursday.
The opinion poll carried out after Mr Griffin's appearance found 22% of voters would consider voting BNP in a future local, general or European election.
Two-thirds of the 1,314 people polled by YouGov for the Daily Telegraph dismissed voting for the party under any circumstances, with the rest unsure.
When asked how they would vote in an election tomorrow, the proportion supporting the BNP stood at 3%, up from 2% a month ago.
However, more than half of those polled said they agreed or thought when asked if the party had a point in speaking up for the interests of "indigenous, white British people".
The poll did not ask whether their views were affected as a result of Mr Griffin's appearance on Question Time.
Mr Hain, who campaigned against Mr Griffin being included in the Question Time panel, said: "This is exactly what I feared and warned about."
Mr Griffin's fellow panellists on the show said he had been "shown up," but critics said the show had given the BNP huge publicity.
The BNP claims 3,000 people registered to join the party during and after the broadcast.
More than 240 complainants to the BBC felt the show was biased against the BNP, while more than 100 complaints were about Mr Griffin being allowed to appear on Question Time.
However, more than 50 people contacted the BBC to show their appreciation for the programme.
Mr Griffin, who was one of two BNP candidates to be elected to the European Parliament earlier this year, faced robust questioning about his views on race, immigration and the Holocaust from a largely hostile audience.
He criticised Islam, defended a past head of the Ku Klux Klan but insisted that he was "not a Nazi".