Twenty-five sitting and former Conservative councillors have been exposed for posting Islamophobic and racist material on social media, according to a dossier obtained by the Guardian that intensifies the row over anti-Muslim sentiment in the party.

The disclosure that 15 current and 10 former Tory councillors have posted, shared or endorsed Islamophobic or other racist content on Facebook or Twitter will increase pressure on Boris Johnson after he backtracked on a pledge to hold an independent inquiry into the issue.

Inflammatory posts recorded in the dossier, which has been sent to the party’s headquarters, include calls for mosques to be banned, claims the faith wants to “turn the world Muslim”, referring to its followers as “barbarians” and “the enemy within”.

In 2017, one councillor, who has been pictured with Johnson, endorsed a suggestion that all aid to Africa helping feed starving people should stop, allowing “mother nature take her course”. She replied: “It’s nature’s way of depopulation.”

The dossier was compiled by @matesjacob, an anonymous Twitter user who campaigns against racism. After being presented with the posts by the Guardian the Conservative party suspended all those who are still members pending an investigation.

The news come days after Johnson made a U-turn on a pledge for the Conservative party to hold an independent inquiry into Islamophobia, instead saying the party would have “general investigation into prejudice of all kinds”.

The cabinet minister Michael Gove had previously said the party would “absolutely” hold an “independent inquiry into Islamophobia … before the end of the year”. It follows repeated warnings about prejudice against Muslims being perpetrated in the party’s ranks and investigations by the Guardian shedding light on Islamophobia in the Conservatives.

Among the series of instances in the dossier verified by the Guardian were posts from:

Councillor Beverley Dunlop. Photograph: Democracy.bcpcouncil.gov.uk

Beverley Dunlop, a councillor in Bournemouth, Christchurch and Poole, who posted messages in two Facebook groups with more than 11,000 members between them. In one posted in 2016 she railed against the burqa, adding: “I hate to ban anything really but I’d suggest we start with Mosques!” In another post, she responded to a call for an inquiry into Islamophobia in the Conservative party by hitting back last year: “How about them calling for an inquiry into Islamist rape gangs grooming underage, underpriveleged white girls [sic]?”

The Walsall councillor Vera Waters who endorsed a suggestion that impoverished Africans should be left to starve, saying that famine is “nature’s way of depopulation”.

Trevor Hales, a parish councillor in Sandiacre, near Nottingham, who complained on Twitter about Muslims in a stream of tweets last year in which he referred to them as “the enemy within”, claimed “spineless” governments had sold “us to slavery of Muslims”, and warned Sajid Javid: “How long are you going to allow this Muslim takeover.”

Malcolm Griffiths, a councillor in Redcar and Cleveland, North Yorkshire, who is also chairman of South Tees Conservative Association, and liked Facebook comments in 2017 urging migrants to “go back to where they came from” and to “get the fuck out and go home”. In a separate post, Griffiths suggested Muslims were inbred.

A Conservative councillor in Kettering, Paul Marks, who referred to London’s Muslim mayor, Sadiq Khan, as a “vile creature” and liked a post ranting about the politician, which claimed he “will always lobby against anybody or anything which finds itself in direct conflict with Islam”.

The post added: “No doubt he will be voted in again by the exploding Muslim hordes that now dominate London and suppress any counter votes from the more white conservative outer London boroughs.” In reply, Marks wrote: “That this vile creature was a elected mayor of London tells me all I need to know about that anti-British city.”

Marks, the Kettering Conservative Association chairman, told the Guardian he regretted liking the post, saying the use of the word “white” was “completely irrelevant”. He added: “After all, Muhammad was probably paler than I am … I certainly don’t agree with [the use of the word] white and I don’t agree with hordes and I never wrote the post.” He said that he likes posts to draw attention to them but does not agree with everything in them.

Facebook Twitter Pinterest Councillor Paul Marks. Photograph: Kettering.gov.uk

Dunlop said the messages she had posted were private.

Sayeeda Warsi, who has been calling for the party to hold an independent inquiry into Islamophobia, said she was appalled by the comments in the dossier. “These further divisive and racist comments by elected Conservative councillors are a further indication of the issue of Islamophobia in the party,” she said.

“The constant argument made by the party [is] that there isn’t the evidence, yet dossier after dossier has been presented to the party. Now this one exposes a sizeable number of sitting Conservative councillors. These individuals seek to represent the party, and if the party truly believes in rooting out racism it should start from rooting out those with racist views from the party.

“Sadly, the party has been trying to downgrade, dilute and deflect the issue of Islamophobia.”

Sajjad Karim, a former Conservative MEP who has spoken out about facing Islamophobia in the party including from a serving minister, said: “The fact that the prime minister has now backtracked on his pledge to hold an inquiry is something that further cements my view that there is no real desire or intent in the party to deal with this issue.”

Waters, the Walsall councillor, whose Facebook profile picture shows her posing with Johnson, endorsed a suggestion that aid to Africa should stop and “mother nature” should “take her course”. In response to an article about UK aid helping fund voluntary family planning in developing countries, a woman wrote on Facebook in 2017: “I suggest ALL aid to Africa stops, ALL immigration from Africa stops – and let mother nature take her course – may seem harsh but it will never end no matter what we do.” In response, Waters liked the post, adding: “I totally agree with you. It’s nature’s way of depopulation.”

Griffiths, the councillor in Redcar and Cleveland, posted an article on Facebook in 2017 that claimed Muslims in Germany had started a petition to end the drinking festival, Oktoberfest, because it was “un-Islamic”. Underneath, a friend of Griffiths wrote: “They can go back to where they came from. Try going to a Muslim country and ask them to stop Muslim traditions because it offends incoming Christians How outrageous is sharia behaviour.” Griffiths liked the post.

In another post in 2017, he quoted Nicolai Sennels, an organiser of the Danish branch of the anti-Islam group Pegida, saying: “The genetic damage done to [the Muslim] gene pool since their prophet allowed first cousin marriages 1,400 years ago is most likely massive.”

A Facebook post by Malcolm Griffiths on the ‘genetic damage’ done to Muslims. Photograph: Facebook

Among the other councillors and former councillors whose racist or Islamophobic messages were seen by the Guardian, the councillor Roger Taylor from Calderdale, West Yorkshire, questioned why prominent British Muslim columnist Yasmin Alibhai-Brown was in the UK. In response to a post about the journalist hitting back at claims she is anti-British, Taylor wrote last October: “Why is she even in the country?”

When approached for comment, Taylor said he would be binning the Guardian’s email. “We are in an election and you are raising this to deflect from the antisemites in the Labour party,” he said.

A Conservative party spokesperson said: “All those found to be party members have been suspended immediately, pending investigation. The swift action we take on not just anti-Muslim discrimination, but discrimination of any kind is testament to the seriousness with which we take such issues.

“The Conservative party will never stand by when it comes to prejudice and discrimination of any kind. That’s why we are already establishing the terms of an investigation to make sure that such instances are isolated and robust processes are in place to stamp them out as and when they occur.”