Muslim leaders from around the world have accused the mainstream media, politicians and academics of contributing to the conditions fuelling terrorist violence against their community such as Friday’s attacks in Christchurch, New Zealand, which claimed 50 lives.

More than 350 leading Islamic figures from countries including the UK, US and South Africa have signed a letter to the Guardian, which links the actions of the suspected shooter, the 28-year-old Australian Brenton Tarrant, to an atmosphere of “systemic and institutionalised Islamophobia”.

The letter says: “This bigotry has been fuelled by certain callous academics, reckless politicians as well as media outlets who regularly feature those who demonise Islam and Muslims with impunity, disguising their vile mantra behind a veneer of objectivity.

“The massacre of Muslims did not just begin with bullets fired from the barrel of Tarrant’s gun. Rather it was decades in the making: inspired by Islamophobic media reports, hundreds and thousands of column inches of hatred printed in the press, many Muslim-hating politicians and unchecked social media bigotry.

“Muslims have been constantly cast as suspect communities, foreigners with barbaric views who are a threat to our society. We are now reaping the awful outcome of systemic and institutionalised Islamophobia woven into many sections of our societies.”

The letter came as the Conservative peer Sayeeda Warsi called on ministers to heed demands from Muslim leaders for an increase in funding for mosque security after the New Zealand terrorist attack. She said Islamophobia was the party’s “bigotry blind spot”.

The former Tory party chair said there was overwhelming evidence to support an increase in funding for mosque protection following the Christchurch attacks.

The Muslim Council of Britain has called for increased funding for Muslim places of worship amid a rise in the number of suspected far-right incidents across England, some of which are alleged to have been inspired by the atrocities in New Zealand.

Lady Warsi made the comments as a row continued over the Conservatives’ record on tackling Islamophobia. Earlier this month, Warsi repeated calls for an internal inquiry and suggested the most senior figures in the party – including Theresa May – needed to take the problem more seriously.

Responding to claims that mosques were inadequately funded, Warsi said: “That’s not an opinion, that’s a fact. There are two different pots of funds: there’s a fund specifically for the protection of synagogues and then a fund of about £2.4m for the protection of all other religious institutions.

“The government has to go back and recognise that there is a need right now – and there’s overwhelming evidence that there’s clearly a need – and therefore how quickly is it going to respond to that.”

But she added: “I’m really cautious about making comparisons. Each form of hate is unique and specific and has its own challenges. It was quite right for thegovernment to respond when they identified a form of hate with antisemitism and made sure places of worship were protected.”

Warsi said a change the government should make – to show it has learned from the attacks in New Zealand – is to adopt a formal definition of Islamophobia. A definition put forward by the all-party parliamentary group on British Muslims had been adopted by the Lib Dems, was being considered by the Labour party and was backed by councils, academics, 850 Muslim organisations and about 70 parliamentarians, she said.

“Whether it’s in relation to the protection of mosques, whether it’s in relation to engagement with British Muslim communities, whether it’s in relation to acknowledging the level of hate either within the party or within government policymaking, whether it’s the way we use the language of British Muslim communities, it is our bigotry blind spot.

“It comes back down to the fact that we fail to see it and recognise it as a specific form of racism that it is, directed at British Muslim communities, and we therefore fail to make adequate policy and respond.”

Harun Khan, the secretary general of the Muslim Council of Britain, will be writing to May raising the “palpable sense of fear” felt by Muslim communities after the attack in Christchurch, and warning of the risk of copycat attacks in the UK.

The government increased security funding for Jewish institutions after the rise in antisemitic attacks, pledging £14m to support the security at about 400 synagogues and 150 Jewish schools, equivalent to more than £25,000 per institution.

Khan called on the government to demonstrate equivalent support to Muslim communities.

The former integration tsar Dame Louise Casey warned that a New Zealand-style atrocity could occur in the UK. Casey said Britain needed to “wake up” as extremism had not been tackled sufficiently.

She also strongly criticised the “appalling behaviour of far too many politicians” and the “mood music” they were creating.

The security minister, Ben Wallace, said: “The funding for protective security tacks with the threat. As the threat changes that funding will change with it.

“In response to increased threat, we will increase and seek to change the funding around that, and that is why we do have the places of worship scheme. We will absolutely be looking at seeing whether that needs to be increased over the short and longer term.”