Ms Andreasen, 58, said resigning from UKIP to join the Conservatives was not a decision that she took lightly.
In an open letter expressing her dissatisfaction with UKIP, she accused Mr Farage of treating any views other than his own with contempt.
Ms Andreasen said she did not believe the party could offer "real leadership".
She said Mr Farage surrounded himself with "an old boys club of like-minded sycophants", whereas Conservative leader David Cameron had shown "he can listen, adapt and do what is right for the country, not just for personal gain".
www.bbc.co.uk/news/uk-politics-21555727