More than a dozen Conservative councillors who were suspended over posting Islamophobic or racist content online – with some describing Saudis as “sand peasants” and sharing material comparing Asian people to dogs – have had their membership quietly reinstated, a Guardian investigation has found.

The chairman of the Conservative Muslim Forum, Mohammed Amin, called on the party to publish a set of formal disciplinary processes after the Guardian found 15 examples of politicians who posted content that was deemed objectionable.

The findings come amid growing concerns over the Conservative party’s attitude to reports of Islamophobia in a febrile wider climate, with the number of hate crimes against Muslims reported to have risen by 593% in the week after the attack on two New Zealand mosques.

The Labour mayor of London, Sadiq Khan, told the Observer on Sunday that he had been repeatedly subjected to anti-Muslim abuse from the Tory party’s members and supporters.

James Cleverly, the deputy Tory chairman, has said he disagrees “deeply” with claims from within the party that it is institutionally Islamophobic. On Sunday a spokesperson told the Guardian that the party “acted swiftly, suspending members and launching immediate investigations, in sharp contrast to other parties” when shown evidence of racism.

But after dozens of members were suspended over alleged anti-Muslim comments last week, the Guardian found evidence of five more apparent members who have posted Islamophobic comments online – referring to Muslims as “cavemen” and claiming “most” of the religion’s followers are sex offenders.

The party’s investigation process will come under severe scrutiny after the disclosure that 15 councillors were reinstated despite apparently Islamophobic or racist remarks. The Guardian’s research suggested that in the majority of cases where a councillor was reprimanded for retweeting or sharing offensive content, they were later readmitted to the party. The party said that selective cases did not provide a complete picture of how complaints were dealt with.

This week, the Conservative leader of Swale borough council, Andrew Bowles, was readmitted to the party after a 13-day suspension for retweeting a post that described the far-right leader Tommy Robinson as a patriot, condemned the decision by Facebook and Instagram to ban him and urged people to retweet if they agreed.

He said he had retweeted the post because he supported Robinson’s “right to have his say” and told the Guardian he did not “have any racist or Islamophobic views”.

In June 2017, the Pendle borough council councillor Rosemary Carroll was suspended for three months for sharing a racist joke on Facebook that compared an Asian person to a dog. She claimed she had shared the post by accident. She was allowed back into the Conservative group on the council on the day of the local election count, meaning the party took control of the local authority by one seat.

Rosemary Carroll shared a racist joke on social media. Photograph: PendleConservatives/Facebook

A councillor on Dover district council, Bob Frost, quit the party last year after being suspended for a third time for offensive social media posts. He was suspended in 2011 after he joked with another Facebook user about the London riots: “I can assure you that all the bunnies here are of the woodland variety rather than jungle,” he said. He later said he had been referring to “the urban jungle”.

In 2015 he was suspended for joking that he had told a Big Issue seller to “f**ck off back to Romania”. Later that year he described Saudi men as “sand peasants” in a tweet.

He was suspended again in May and told the Guardian in an email: “I have indeed now left the Conservative party as I didn’t wish to waste either mine or their time with suspension (which is apparently de rigueur nowadays when anyone claims someone has been using hurty words).” He described the situation as “a load of bollocks conjured up by a load of humourless lefties”.

The Solihull borough council councillor Jeff Potts was suspended in September after retweeting a post that said Muslims should to be deported or terrorists would “kill innocent people for generations to come”. A further retweet read: “You’ve clearly not experienced the Pakistani hospitality, having a daughter raped by men who think she’s ‘white trash’.” In November it emerged he had been readmitted to the party, though he is still sitting as an independent on the council.

Amin said his party had historically been slow in “coming to terms with the way our country had changed over the last 60s years”. “There is a problem, in my view, particularly at grassroots level, with far too many people who have absorbed anti-Muslim bigotry, because there is quite a lot of it around,” he said.

“In terms of terrorism carried out in the name of Islam – 9/11, 7/7 etc – it’s quite easy to see where these anti-Muslim ideas come from. But the party as a whole, in my view, hasn’t succeeded in getting to grips with that.”

“There is one fundamental change that is required,” Amin said. “There needs to be a more transparent process, much clearer criteria, which are published. Not just criteria for suspending people but criteria for unsuspending people.

“I personally don’t necessarily have a problem with unsuspending people who have apologised and undergone diversity training, for example. But what we need is a clear published rule book.”

Miqdaad Versi of the Muslim Council of Britain said it seemed the party ‘wants to sweep this issue under the carpet’. Photograph: Martin Godwin/The Guardian

A Conservative spokesman said: “Discrimination or abuse of any kind is wrong. When CCHQ has been made aware of the small number of such cases we have acted swiftly, suspending members and launching immediate investigations, in sharp contrast to other parties.

“Our complaints process is rightly a confidential one but there are a wide range of sanctions to challenge and change behaviour, including conditions to undertake training, a period of suspension and expulsion, and these are applied on a case-by-case basis.”

Not all complaints are made to CCHQ (Conservative Campaign Headquarters), and some cases are handled locally.

Other examples of councillors readmitted to the party include the Calderdale councillor Mike Payne, who was suspended after it emerged he had posted an article entitled “France slashes benefits to Muslim parasites by 83%” on his Facebook and Twitter accounts in 2013 before he was a councillor. He has since been reinstated.

The article said France was “no longer able to hold back the crowd of Muslim invaders, many of whom do not want to integrate into the social and economic life of their new home country, and are content to live off the state and breed like rabbits”.

Miqdaad Versi, assistant secretary general of the Muslim Council of Britain, said: “It’s clear to me that the party wants to sweep this issue under the carpet and it cares more about the public image than stopping racism have a hold in the party.”

“At the very minimum they need to have an independent person look at this. I don’t think people really have trust that the chair of the party has any handle on the situation whatsoever.”