A gay researcher for Cornwall MP Candy Atherton has claimed his former boss asked him to "dig the dirt" on a gay Conservative political opponent.
Candy Atherton "vigorously" denies the claims
Paul Phillips, 36, told a tribunal the Labour MP for Falmouth asked him to use his contacts to find out whether Ashley Crossley frequented gay bars in Soho.
Mr Phillips later resigned from his job and started discrimination proceedings.
Ms Atherton has pledged to "vigorously" contest the case being heard at an industrial tribunal in south London.
Mr Phillips said: "(I was) being asked to dig the dirt on Mr Crossley.
"Facts about Mr Crossley's sexual orientation and lifestyle could only be used to damage him. There could be no other reason for wanting this kind of information.
"I felt very much on the spot being asked to check on a fellow gay man's lifestyle, especially someone standing for Parliament in Cornwall, a fairly homophobic area."
The 36-year-old, from Wapping, east London, told the hearing in Croydon that Ms Atherton approached him one day with a "research project".
"She said she had been doing some research and had found out that Ashley Crossley, the Conservative prospective parliamentary candidate selected to stand against her, was gay and visited bars in Soho looking for men," Mr Phillips said.
"I said 'I can't do anything which is homophobic.' Candy said she wasn't going to do anything that was homophobic but she just believed in people being open about what they did."
Mr Phillips, who is claiming discrimination on the grounds of sexual orientation, was employed by Ms Atherton for 12 months until he resigned in March 2004.
He said: "I believe that the only reason Candy asked me to check on Mr Crossley's lifestyle was because I was a gay man.
"I do not believe that this was a normal part of a research assistant's job or that she would have asked a straight man or woman to do it.
"I felt humiliated that she should expect me to behave like this."
The hearing continues.