A leading academic has called on the Church of England to publish details of the "proven growth formulae" on which a £100 million recruitment plan is based.

The Church Commissioners have been accused of raiding church assets and adopting a "back-of-the-fag-packet" solution to church decline.

Professor Linda Woodhead, of Lancaster University, who specialises in the sociology of religion and also organises the Westminster Faith Debates, told Christian Today that she believed the proven growth forumla to be a "nonsense" that "needs exposing". She added: "If they have a worked business case, why can't we see or debate it?"

She has written to William Fittall, secretary general of the General Synod, the Church's governing body, asking to see the growth formula. "This should help clarify whether the back-of-the-fag-packet charge is unfair or not," she wrote on Facebook.

The Financial Times this week reported criticisms that the Church's £100 million plan to address a relentless decline in membership amounted to "spending the family silver".

The Church Commissioners manage the Church's £6 billion assets and the plan is to sell some of these investments in order to recruit more clergy. A further £2 million has already been set aside for the talent management programme proposed in a report earlier this year that chaired by the Rev Stephen Green, a church clergyman and former chairman of HSBC.

Andreas Whittam Smith, the First Church Estates Commissioner, wrote on the Church's Tumblr blog that investments and spending were traditionally managed to preserve the real value of the capital over time, and suggested this should now change. "The question now arising is whether this admirable rule can apply when the membership of the Church is shrinking. One doesn't want to arrive in a situation when a small Church of England has a huge endowment. Our successors might then wonder why we hadn't used our financial strength to arrest the decline when there was still time."

He also said the Church had to face the "terrifying" facts about its membership. "The church has got older; the average age is approaching 70 I believe, and these people die off, but you have to have the same number of younger people joining just to keep level, let alone grow. For it to grow you have to have more younger people joining, and that's the missing bit at the moment, something has to happen."

Eve Poole, who had previously worked for the Church Commissioners and is now employed by Ashridge Business School, told the FT: "My concern is that I think this is a back-of-the-fag-packet solution... These are big numbers. It's quite a raid on assets without much due process."

A conservative MP has also tabled a question in Parliament. Professor Woodhead said she believed the plan simply would not work as the numbers of churchgoers had been declining for 80 years even when there were far more clergy than at present.

The Church has described the proposals as "targeted funding seeking to fund best practice of implementing proven growth formulae."

A Church spokesman defended the proposals to Christian Today. He said: "Last month, General Synod overwhelmingly supported a motion for the Church Commissioners to give consideration to releasing additional funds in order to support changes that will equip the Church of England more effectively for sustainable mission and ministry over the coming generations. The Commissioners' Board of Governors has also expressed its preparedness to release additional time limited funds provided this could be done in an effective and disciplined manner. Robust processes are already in place to govern the distribution of existing discretionary funding streams and the same will be true of any additional funding."