An MP’s chief of staff raped a woman in the Houses of Parliament after a night of drinking, a court has heard.

Samuel Armstrong was accused of attacking the woman in the office of his boss, the Conservative MP Craig Mackinlay, last year.

The jury heard that after the attack, the woman fled through the Palace of Westminster before asking a cleaner to call police.

During cross-examination, the woman denied the defence’s suggestion that she had consented to having sex with the aide, then changed her mind.

Mark Heywood QC, prosecuting, said: “On a night in the autumn of last year, this defendant abused his position – his position as someone newly in charge of other people. And, after an evening drinking at his workplace … [he] took advantage of her.”

Armstrong was charged with two counts of sexual assault and two of rape. “Each of those charges reflects things that happened to her – that he did to her,” Heywood told Southwark crown court in London.

The jury was played a recording of a police interview with the woman, who cannot be named for legal reasons, in which she described the attack.

She told an officer she froze after Armstrong started to kiss her. She said she explicitly declined an invitation to go back to Armstrong’s flat, but he undressed her and raped and sexually assaulted her.

She said he told her “this is what you want”, which made her feel there was an “inevitability” that they would have sex, despite her not wanting to.

The court heard the pair had been drinking in the Sports and Social bar in the Houses of Parliament and, at one point in the night, Armstrong took her to the roof garden terrace so she could hear Big Ben chime. Later, the jury heard they went with two other people to the Lords’ offices, before Armstrong and the woman headed alone to Mackinlay’s office.

CCTV footage shown to the jury showed them in Westminster Hall. The prosecution said: “It is obvious that the mood between them was light-hearted and playful.”

Once inside Mackinlay’s office, “it began to go wrong”, Heywood said. “The defendant took gross advantage of the situation and of her obvious and open friendliness towards him.”

Armstrong became insistent and determined, the prosecutor continued. “As he knew perfectly well, he had her [at] a very distinct disadvantage. His manner was now changed. He repeatedly called her a ‘bitch’,” he said.

CCTV footage from later in the evening was also shown to the jury, and the prosecution said it showed the woman “moving in an agitated way, neither relaxed nor at all happy … her steps are purposeful; she looks around herself and behind her at times, and her step breaks into a little run from time to time”.

Questioned later in the proceedings by Sarah Forshaw QC, for the defence, the woman denied having begun to find Armstrong attractive as a result of being “disinhibited by drink” and having a “fascination with parliament”.

She agreed that he had been “sweet” towards her in the time they had known each other, but denied that her distress that night was due to her having got lost in the parliament buildings after leaving Mackinlay’s office. “I couldn’t find a way out and I was, absolutely, crying, because a man had forced himself upon me,” she told the jury.

Forshaw suggested the sex between the pair had been consensual. “[It is] 2am and you can’t find a way out and you get more and more anxious and you have been with Samuel Armstrong until 2am. You have got to go home and you have told your boyfriend you wouldn’t be out late,” she said.

“I am going to suggest to you that that is when you decided that you had not wanted what had happened in the office.”

The woman denied the suggestion. Armstrong denies the charges. The trial continues.