The Conservative party chairman, Brandon Lewis, has been accused of ignoring repeated pleas by Tory members to investigate alleged racist and Islamophobic incidents, including a claim that a white senior party official referred to an Asian councillor as an orangutan.

Lewis, a key ally of Theresa May, wrote to party officers earlier this month to reassure them that he would “continue to take a zero-tolerance approach to any form of discrimination, intimidating behaviour or abuse”.

It followed allegations from the Tory peer Lady Warsi that the party is institutionally Islamophobic.

But Conservative party members in the Portsmouth South constituency in Hampshire have told the Guardian that Lewis has failed to acknowledge repeated requests to investigate a dozen alleged racist and Islamophobic incidents.

Registered letters of complaint sent to his office had been ignored, activists said.

Local members have written to Lewis and local party officials claiming there is “compelling evidence” that minority ethnic members were being bullied, racially abused and cut out of communications to ensure they were not chosen to represent the party.

In one alleged incident, a senior named party officer was accused of referring to a Conservative council candidate of Indian origin, Prab Ghosh, as “Oran”, allegedly a shortening of “orangutan”.

Another white local party official was accused of aggressively challenging the right of a prominent female Muslim member to attend a meeting, even though the woman, a local GP, had been a member for several years.

Racial tensions within the constituency party have risen over the past five years as an increasing number of black and Asian members living in the south of the borough have sought to become party officers, local activists claimed.

Last year, at least four city council candidates were of BAME origin. However, the number of candidates from minority ethnic backgrounds is expected to fall after the constituency party’s executive ruling body, including several officers with a BAME background, was ousted in September.

Massoud Esmaili, a Tory activist for 10 years who stood to become a city councillor, has written to Lewis outlining a number of allegations of racism but says he has yet to receive a reply.

Esmaili, 67, who came to the UK from Iran 40 years ago and whose daughter is a major in the British army, said he had faced prejudice sporadically from fellow Conservatives over the past decade – on one occasion, a councillor called him “a bloody foreigner” and told him to “go back to your own country” – but prejudice within the party was at its height, he said.

“It is as if the party has gone back to the 40s or 50s. People in the party feel able to be as racist as they wish now,” he said.

He wrote to the local party last year saying that potential council candidates were subjected to racial profiling before being given updates on future meetings.

“Whilst standing as a council candidate I and other BAME members were denied updates to meetings with the council leader. Other candidates with different racial profiles were welcomed,” he wrote.

“It is glaringly obvious and there is compelling evidence that BAME members are treated differently than others. I again demand that an independent investigation is carried out and our serious concerns and complaints are addressed.”

The ousted chair of the Portsmouth South constituency, Leo Ciccarone, said he first sent a letter to Lewis alleging racism in the party two years ago. Since then he had sent another two letters to the party’s chair by registered post, he said, as well as numerous emails.

In one email, he wrote: “There are some dozen outstanding racism complaints. These include a particularly shocking incident, where [a named party official] aggressively challenged a female Muslim member about her right to be at a meeting of the executive and how long she has lived in the UK and another incident involving [another party official] where he either deliberately, or through gross incompetence, insulted three BAME members.”

Ciccarone, 70, who is of Italian origin, said Lewis was misleading the public when he claimed that he investigated all allegations. “Our local party is riven with racism, from top to bottom. Lewis has blatantly ignored my pleas to investigate,” he said.

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Lewis wrote to all of the party’s regional, area and association “chairmen” on 6 March claiming to have taken a “zero-tolerance approach” towards any form of discrimination and pledging to investigate all claims.

He said: “I continue to take a zero-tolerance approach to any form of discrimination and abuse within our party and have taken swift action to suspend members or refer cases to the code of conduct for investigation when and where I see it.

“We have a long and proud history of delivering opportunity for all, and this belief is the driving purpose of our party. We are at our best when we reflect the fantastic diversity of society and it is unacceptable that anyone should face discrimination or abuse because of their faith, race, gender, sexuality, disability or ethnicity.”

Lewis has been approached for a response.

Following publication, a Conservative spokeswoman implied that the allegations were subject to disciplinary proceedings, but did not respond to requests to explain further. “We do not comment on ongoing disciplinary matters,” she said.