A rare expression of internal dissent has broken out in the world of Scientology, with one of its most senior figures sending a mass email to 12,000 fellow members complaining that the self-styled church has become obsessed with fundraising and has amassed unused reserves of over $1bn (£640m).

Debbie Cook says she remains committed to the group, set up in the 1950s by the former science fiction writer L Ron Hubbard. She is critical, however, of David Miscavige, who has led the Church of Scientology since Hubbard's death, 26 years ago.

Miscavige, who has faced and vehemently denied accusations of a despotic leadership style and physical assaults on colleagues, has presided over a "new age of continuous fundraising", Cook claims.

In what appears to be a direct attempt to undermine Miscavige's leadership, Cook, who spent three decades inside the church's equivalent of a clergy, the so-called Sea Org, , urged people receiving the email to reject orders not directly backed up by Hubbard's teachings, and to encourage other members to do the same.

One recipient passed the email to New York's Village Voice, which posted it in a blog. While the email was later removed at the request of Cook – who has verified its authenticity – it has been republished elsewhere.

Under Miscavige's leadership, Scientology has expanded hugely, not least thanks to celebrity endorsements from the likes of Tom Cruise and John Travolta. However, the church has faced criticism for alleged pressure on members to donate regular and sometimes significant sums of money.

An obsession with money, said Cook, was both contrary to Hubbard's teachings and a distraction from efforts to disseminate the late writer's beliefs. These involve psychoanalysis-based self-help and counselling, and the supposed malign influence of alien spirits brought to Earth 76m years ago by a galactic emperor called Xenu.

In an email full of church jargon and Hubbard references, Cook quoted the church's founder as decreeing that the maximum sum people should pay to be a Scientologist was a lifetime membership of $75. Instead, under Miscavige, Scientology had acquired a vast and largely unused wealth, she said.

"Currently, membership monies are held as Int reserves and have grown to well in excess of a billion dollars. Only a tiny fraction has ever been spent, in violation of the policy above. Only the interest earned from the holdings have been used very sparingly to fund projects through grants."

Miscavige had also been dismantling the church's original management structures, Cook said, writing that she had spent time working at an otherwise empty headquarters building: "empty because everyone had been removed from post. When I first went up lines I was briefed extensively by David Miscavige about how bad all of them were and how they had done many things that were all very discreditable. This seemed to 'explain' the fact that the entirety of the Watchdog Committee no longer existed."

Miscavige had dismantled the "complete and brilliant organisational structure" put in place by Hubbard, she added. "There never was supposed to be a 'leader' other than LRH himself as the goal maker for our group."

Cook urged fellow members to refuse to make new donations unless church authorities could produce a Hubbard teaching to say they were required, adding: "No one will be able to produce any references because there aren't any." She went on: "The other thing you can do is to send this email to as many others as you can, even if you do it anonymously. Please keep this email among us, the Scientologists. The media have no place in this."

This is a deeply awkward moment for the Scientology organisation, which has been attempting to regain the initiative after a period that has seen the public defection of another Hollywood adherent, the Oscar-winning director Paul Haggis, as well as the allegations against Miscavige and judicial investigations in France and Australia.

In a statement sent to the Times, which followed up the Village Voice blog, the Church of Scientology dismissed the email, saying: "Ms Cook's opinions reflect a small, ignorant and unenlightened view of the world today. They are not shared by the thousands of Scientologists who are overjoyed by our 27 new churches and what they mean to the communities they serve."