Professor Linda Woodhead, of Lancaster University, who specialises in the sociology of religion, told Christian Today that she believed the "proven growth forumla" cited by the Church in an article in the Financial Times was a "nonsense" that "needs exposing". She added: "If they have a worked business case, why can't we see or debate it?"

She wrote to William Fittall, secretary general of the General Synod, the Church's governing body, asking to see the growth formulae after the Financial Times 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 £6bn assets and the plan is to sell some of these investments in order to recruit more clergy. A further £2m 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.

Mr Fittall, who recently announced his retirement, said in his response to Professor Woodhead that "established evidence about growth" would have been a better phrase than "proven growth formulae".

He noted that the General Synod in February had agreed that it was justifiable to spend the Church's historic endowment to try and reverse the cycle of decline. On the question of whether the expenditure would actually work, he said it was preferable to keep the debate to "what will work best" rather than whether anything works.

He said the "established evidence" showed that growing churches were those with good leadership, clear mission and purpose, reflective practice, change and adaptivity, involvement of laity and with a focus on growth and leadership. He also warned that because of the retirement bulge of baby boomer clergy, numbers will fall "significantly" over the next ten years without recruitment being stepped up.

Professor Woodhead said she remained "at a loss" to know how a 50 per cent increase in clergy recruitment and training, to be funded by the sale of historic assets, will lead to church growth. "To base a huge expenditure on such an unevidenced proposition seems reckless in the extreme."

Meanwhile, in response to a question tabled in Parliament about the sell-off, Sir Tony Baldry, MP for Banbury who represents the Church Commissioners, said that synod had backed the idea of the Church Commissioners "giving consideration to the basis on which they might, for a limited period, release 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."

Synod had also agreed that extra funds could be released provided it could be done in an effective and disciplined manner.

He said: "The specific funding criteria have not yet been worked out but I can assure my hon. Friend that any additional money will only be released for focused, evidence-based projects which are rooted in dioceses' strategies and will be advised by carefully worked through calculations by external actuaries as to the impact on the Commissioners' ability to make distributions to the Church in perpetuity."