The Royal Mail is investigating claims that its staff have dumped far-right election leaflets "across Wales" instead of delivering them to households.
BNP chairman Nick Griffin has accused postal workers of dumping hundreds of leaflets at a single address and claims the party has "heavy reports" of widespread non-delivery of its election pamphlets.
"These have been dumped undelivered, likely across the whole of Wales," he told members in an email.
"Royal Mail [has dumped] 400 of our Welsh Local Assembly election leaflets in the back garden of one of our candidates," he claimed.
"It would seem that they have "lost" many of them," he continued.
"So we now urgently have to re-print them and take them back to the delivery offices."
A Royal Mail spokesman said: "We are aware of this complaint which has not yet been substantiated. However, we are now investigating accordingly."
During previous election campaigns, some postal workers have refused to deliver BNP leaflets using a "conscience clause" in their contracts that allows them to reject literature they find offensive.
In 2009 around 100 delivery staff in Bristol and Somerset refused to handle pamphlets with an anti-immigration message.
The Royal Mail has legal obligations under the Representation of the People Act to deliver election material, meaning that in theory it would have to make alternative arrangements to deliver any leaflets its staff declined to deliver.
Kevin Slocombe, communications director at the Communication Workers' Union, told Sky News: "At this point in time we have absolutely no idea as to the details or validity of those claims."