“It is not yours, it is God's, and you are not going to get it.” So saith Kenneth Copeland, the television evangelist, when asked to submit his ministry's private financial records to Washington.

Mr Copeland is one of at least six American “televangelists” facing the scrutiny of a senate investigation for alleged financial wrongdoing. The Eagle Mountain International Church - otherwise known as the Kenneth Copeland Ministries - preaches a doctrine of financial prosperity, with the promise that God can make a follower both healthy and wealthy. The faithful are encouraged to dig deep and give to the Church, where donated dollars will provide a one hundredfold return in happiness and wealth.

As Mr Copeland's televised congregation listen to their minister boom, “You are not created for poverty,” they deposit cash in a donation envelope across which is written: “I am sowing $____ and believing for a hundredfold return.”

Mr Copeland certainly practises what he preaches. According to a report into the pentecostal charismatics, commissioned by the Senate, the ministry built Mr Copeland and his wife Gloria a mansion “the size of an hotel” and enabled him to acquire a $20 million (£10 million) Cessna Citation to help him to spread the word of God across the US.

Speaking to his assembled congregation on the runway by his new aeroplane, Mr Copeland said: “The Lord spoke to me and said ‘you're gonna believe for a Citation 10, right now'.” He also promised that the jet, one of four owned by the Church, “will never ever be used as for anything other than what is becoming of you Lord Jesus”.

The ministry also owns an airport capable of accepting jet landings, leases land for Mr Copeland's cattle and horses and also leases land to the family so that it can operate oil and gas wells.

Last year Senator Charles Grassley, a Republican member of the Senate Finance Committee, began an inquiry into at least six televangelists and commissioned a report by the Trinity Foundation, a firm of private investigators who specialise in religious fraud. The Senate has requested financial information from the churches, and broadly, they have refused to comply.

Mr Grassley is worried that the church ministers may be over-compensating themselves, diverting church funds, or trying to use a non-profit organisation to run an ordinary business. While the churches have complained that they have been unfairly singled out for scrutiny and have accused Mr Grassley of McCarthyism, the real row centres not around religion, but around tax. Under US law, there are many institutions which enjoy tax-exempt status, such as private charities, certain academic foundations and religious institutions. Most of them are required to disclose details of their finances, such as executive compensation, annual revenues, profits, assets and total operating costs. Churches however, are not required to disclose any details of their finances.

When Mr Grassley launched his investigation, he quipped that Jesus rode into Jerusalem on a donkey, not in a Rolls-Royce, but insisted that the purpose of his inquiry was to make sure that the churches were behaving as non-profit organisations and that they were complying with tax laws.

He said: “They are non-profits, like anybody else I have looked into. I have sent them some letters because I want some information.”

While a spokesman for Mr Grassley told The Times at the end of last week that the senator is hoping that the inquiry will force the churches to adopt “self-governance reforms”, the tone of the Trinity Foundation report is more extreme.

In the report, its author, Ole Anthony, writes of these types of churches: “Simply put, a massive amount of money given by well-meaning donors is intercepted by the organisation's hierarchy and often never reaches the people who were to benefit according to the non-profit charter. The problem ...is exacerbated by a lack of disclosure and transparency as well as understaffing with the exempt organisations division of the IRS [Internal Revenue Service].” The report also asserted that “in most of these cases only a miniscule part of the organisation is related to religious worship”.

While Shane Hamilton, the lawyer representing Mr Copeland and his church, declined to comment, the ministry insists that it has done nothing wrong and has pointed out that other than demands from the senate committee, it is facing no other allegations or inquiries regarding any financial irregularities.

Mr Copeland has instead invited the US tax authorities to conduct an audit. It is not yet known whether the IRS intends to conduct its own inquiry. One advantage of an IRS audit is that all such tax inquiries are strictly confidential, and would prevent details of the churches' finances becoming public.

In a television interview last month, Mr Copeland's son John hit back at allegations of financial impropriety:

“The jet is a tool. It is just a tool to use in ministry. Where in the Bible does it say you should have watchdogs and judgment groups that watch over ministries?”