Anthony d’Offay, one of the most powerful figures in the contemporary British art world, is facing allegations of sexual harassment and inappropriate behaviour from three women with whom he has worked.

The 78-year-old dealer created the Artists Rooms project after giving much of his multi-million-pound collection to the Tate 10 years ago. The Observer has also established that police are investigating d’Offay after receiving a complaint from a young woman that he sent her malicious messages.

D’Offay stepped down on 19 December from his role as ex-officio curator to the Artist Rooms, jointly owned and managed for the nation by the National Galleries of Scotland and the Tate – although this has yet to be announced.

The allegations of sexual harassment and inappropriate behaviour date from 1997 to 2004 and come from women with successful careers in the art world. The women say they feel duty bound to speak out, and believe their action will encourage others to come forward. D’Offay strongly denies the allegations and says he is unaware of a police investigation.

Profile Who is Anthony d'Offay? Show Hide Anthony d’Offay is used to making headlines in the art world – first, in 2001, when he suddenly announced he was to retire after more than 30 years as one of London’s elite fine art gallerists, and then again when he was applauded up and down the country for his decision to give his £125m personal collection of postwar works to Tate for only £26m and to the National Galleries of Scotland for nothing, as part of a reciprocal tax arrangement. The collection forms the basis of a series of 50 free touring exhibitions called Artist Rooms that have been seen by almost 30 million people. At the time it was described as a “supreme act of philanthropy”, welcomed by Gordon Brown and by Sir Nicholas Serota, now head of the Arts Council England, who said the act of “imaginative generosity” was “without precedent anywhere in the world”. Born in Sheffield to a French father, d’Offay was only 25 when he opened his first tiny gallery near Piccadilly. Within a year he had sold a drawing by Jean Cocteau to Paul McCartney and he launched a new gallery near Bond Street in 1969. He helped revive the reputations of Wyndham Lewis and Stanley Spencer and in the year his gallery closed it hosted exhibitions of Ron Mueck, Anselm Kiefer and Bill Viola. Vanessa Thorpe Photograph: David Cheskin/PA Archive

One woman, a former employee who joined his London gallery in 1998 aged 25 and was asked by him to become an assistant, is in breach of non-disclosure provisions of a settlement agreement.

“He started taking me for meetings and appointments outside the gallery. He would hold on to my arm or put his arm around me. I thought it was not quite right, but dared not voice my discomfort,” she said. Her concerns deepened when he informed her that he would like her to accompany him to New York. “That’s when things escalated. He grew more touchy, and would put his hand around my waist, very close to my bum. There was no sense of boundary in respect to personal space.”

Events came to a head in October 2000 at his former gallery in Dering Street, when she says he approached her while she was on the phone. “He grabbed me. Pulled me really tight and started kissing my neck. I pushed him away. Because I was on the phone I could not scream. Pushing him away was the only thing I could do,” she said. The incident, she says, was captured on CCTV. She signed a settlement agreement and left his employment shortly after.

Another woman said she was introduced to d’Offay, when she was 34, as a possible mentor, but felt increasingly uncomfortable. She says he began to phone her outside work, with calls increasing in frequency. On the final call she could hear water and says that she realised he was in the bath. She says that his voice slowed, his breathing became heavy and she believed he was masturbating. “I felt violated that he had used my voice to service his sexual needs. I genuinely thought I was having a phone conversation about a high-profile German artist. I was in complete shock when I realised.”

Prior to the final call, she was invited to his house for dinner, with others, in the middle of 2004. “After dinner it was suggested some guests go for an evening stroll. As we reached the park gate, suddenly, the other guests said they felt unwell and went home.

“I felt very uncomfortable about the fact that I was in the park alone with him. He said: ‘You are being very cruel by not reciprocating my feelings.’ My first reaction was to laugh, ‘You’re married and in your 60s – this is ridiculous.’

“I really needed to get out of the park, but he lunged at me with his mouth open. I was in disbelief. I pushed him away and shouted loudly, ‘No, Anthony, absolutely no!’

“He laughed and said that I needed to surrender to him and in time I would. I did not feel I could tell anyone about this because he was one of the most powerful men in the art world.”

She relates another incident, a meeting at his gallery, with others present, where he offered her an Andy Warhol show. “I said in the light of what has happened I don’t think I can work with you and, with respect, I am turning down the Warhol show. He said, ‘Right, you can go out the tradesman’s entrance, that is where you belong.’

“He called me later and said: ‘Well, you have to mourn us now including all opportunities you have just turned down because of your rejection of me.’ By then I had my family advising me. On my birthday they had witnessed how he had come around to my house and put a rare postcard through the letter box – a drawing of a courtesan on a chaise longue. He had written, ‘Happy birthday. This reminds me of you.’

“After the phone call I refused to have any other interactions. I knew that he would try to punish me professionally and indeed he did for several years afterwards, but at least I had my self-respect intact, even if my business missed out financially. I had not felt able to speak about this. After all, the art world is a very small place and Anthony d’Offay has been a major philanthropist.

“However, in December I became aware of allegations made by the young woman to the police and I felt compelled to speak out.”

A third woman, who worked at d’Offay’s gallery for two years from late 1997, said she made many complaints about how he had spoken to her inappropriately. She was told that he would be spoken to, but believes her complaints were shrugged off and she left the gallery following a confrontation with d’Offay.

“I remember saying he could not speak to me in this way ever again, over something he did in front of a group. It was an artist, a collector, five or six people, and he said something that was thoroughly humiliating. It was something about sitting in one of your laps and giving him a french kiss. It was something suggestive and embarrassing.”

D’Offay said of the allegations by the three women: “I am appalled these allegations are being levelled against me and I categorically deny the claims being made.” He added. “I am completely unaware of any police investigation. If there is one, then police time is being wasted.” He also said: “I conceived the idea for Artist Rooms some 15 years ago. It has been a wonderful success. However, having been directly involved for that length of time and also reaching 78 years old, I decided in December it was time to retire as ex-officio curator.”

A Metropolitan Police spokesman said: “Police received an allegation of malicious communications on Wednesday, 20 December. Officers from the Central North Command Unit investigate. No arrests; enquiries continue.”