The UK Independence Party will file a formal complaint over alleged "abuses" in Thursday's parliamentary by-election in Oldham West and Royton, leader Nigel Farage has announced.
Ukip failed to make widely-predicted advances in the poll, which saw Jeremy Corbyn pass his first electoral test with flying colours as Labour's Jim McMahon held the seat with a majority of more than 10,000.
Mr Farage said he was not questioning Labour's victory, but denounced the electoral process as "bent" after claims that people had arrived at polling booths carrying bundles of postal votes.
The Ukip leader said the result raised questions about the conduct of elections in areas with large ethnic minority communities and claimed that in constituencies with large numbers of minority voters who do not speak English "effectively the electoral process is now dead".
Mr Farage told BBC Radio 4's Today programme: "There are some really quite big ethnic changes now in the way people are voting. They can't speak English, they have never heard of Ukip or the Conservative Party, they haven't even heard of Jeremy Corbyn.
"I'm commenting on the state of modern Britain, post mass immigration. It means effectively that in some of these seats where people don't speak English and they sign up to postal votes, effectively the electoral process is now dead."