A longstanding Liberal Democrat MP is being sued for allegedly sexually assaulting and exploiting a vulnerable constituent who has a mental illness.

Mike Hancock, the MP for Portsmouth South, has been accused of forming an inappropriate relationship with the woman over a 10-month period after she approached him for help with noisy neighbours.

The MP placed his hand on her breast, exposed himself and kissed her on many occasions without consent, it is alleged in high court papers. His actions amount to sexual assault, harassment and misfeasance in public office, the papers claim.

Evidence which may be presented to the court includes text messages sent by Hancock that described the woman as "sexy" and "my princess".

It is claimed that he sent her gifts including House of Commons mints, wine, clothes and perfume. He also gave her a House of Lords teddy bear he named Mike, it is alleged.

Hancock, 66, was arrested over the allegations in 2010 but has vigorously denied sexually assaulting the woman. His solicitors have said that the allegations were examined by the Crown Prosecution Service but were dropped because there was no case to answer.

The woman, 38, who has a borderline personality disorder and is registered as a vulnerable adult, lives with her 15-year-old son in Hancock's constituency. She said last week that she was motivated by a sense of injustice. "He, like Jimmy Savile, has abused his position to take advantage of someone who is vulnerable," she said.

"I want the high court to look at what he did to me and rule on whether a powerful man … should be allowed to use his job to get what he wants."

Harriet Wistrich, the complainant's solicitor, from the firm Birnberg Peirce, said that despite the Crown Prosecution Service's decision not to prosecute, she believed there was enough evidence to convince a judge that her client was a victim of assault and harassment by a public servant abusing his powers. She added that her client had been forced to take her case to the high court because there appeared to be no regulatory framework to deal with complaints about the personal conduct of MPs towards their constituents.

The woman sought Hancock's assistance in October 2009, court papers claim. Hancock, who was appointed a CBE for his work with a mental health charity, wrote to the council on her behalf and made reference to her vulnerable status.

Over the following 10 months, he formed an inappropriate relationship and persistently subjected her to upsetting sexual treatment, it is claimed.

Hancock has previously admitted sending dozens of text messages to her. He is said to have written: "Please give me a chance you never know my princess xxx", "… you are special and sexy to me xxx", "just got here and in bed alone my love xxx". When previously confronted over the texts by a journalist, Hancock said he had called her "sexy" because she had asked him to.

The woman suffered personal injury as a consequence of Hancock's treatment of her, court papers allege, and she is claiming up to £50,000 in damages.

Her legal team say they have become increasingly frustrated by the failure of police and the authorities to investigate her allegations properly. The woman complained to the police in 2010 but the inquiry was dropped. John Lyon, the then parliamentary commissioner for standards, told the woman's legal team in October 2011 that he did not have a remit to examine Hancock's behaviour. The Liberal Democrats were asked to investigate the allegations in 2011, but declined on the grounds that the police inquiry and parliamentary authorities had found no case to answer.

Portsmouth city council, where Hancock is also a councillor, was due to hear the woman's complaint last Friday but the meeting was suspended the day before because Hancock presented a certificate of illness.

Hancock, who is married, has been accused of inappropriate sexual behaviour before. Last year, his former parliamentary assistant Ekaterina Zatuliveter, who was cleared of spying charges, told a criminal court that she had a four-year affair with him. Before another court last year, he said he had a "close and affectionate" relationship with a 17-year-old girl who had worked in his office, but insisted it had not gone beyond kissing and cuddling.

Hancock is expected to deny behaving inappropriately and to contest many of the statements in the claim. He insists that he has never sexually assaulted the woman. His lawyers will say that the police's decision to drop their inquiry in January 2011 was because they did not see the woman as a credible witness.

Hancock has admitted to knowing the claimant for months, sending her texts and taking her on a trip to the Houses of Parliament. Contacted on Friday, he said he was not aware that papers had been filed at the high court and he did not wish to comment on them.

"I don't know anything about this, I haven't responded. I think you need to speak to their solicitors. I am not in a position to speak about this because I don't know anything about the high court. You caught me on the hop. I am not commenting on any of it," he said. His solicitors did not respond to telephone calls.