Respect MP George Galloway is suing the National Union of Students for libel after it labelled him a "rape denier".
Campaigners had earlier criticised his remarks on Julian Assange, who denies sexual assault claims.
He was quoted as saying the Wikileaks founder was accused of nothing more than "bad sexual etiquette".
A spokesman said the NUS comments were "defamatory" and "offensive" and the MP was "absolutely clear that no means no" and "non consensual sex is rape".
The action has been prompted by the NUS description of Mr Galloway and its ban on him from speaking at events affiliated to the union.
A spokesman for Bradford West MP Mr Galloway said: "The NUS can ban whoever they want, but George is categorically not a rape denier."
Mr Assange, 40, is wanted in Sweden for questioning on sexual assault allegations made by two women.
In August, Mr Galloway, who had also said the women's claims were "totally unproven" and the Wikileaks founder had been "set up", was criticised by anti-rape campaigners.
Any damages recovered from his legal action "will be donated to the defence fund for Julian Assange and Bradley Manning".
US army soldier Private Manning is the alleged source of Wikileaks' revelations.
A spokesman for the NUS said it was yet receive a letter from Mr Galloway's lawyers, and would not comment until it had.
Mr Galloway himself was unavailable for comment because he is in Venezuela ahead of the country's presidential elections.