The Church of England's mission in Cornwall is facing an "organisational death spiral" unless worshippers dig deeper into their pockets, a bishop has warned.

Truro is facing a £1 million shortfall this year, in spite of being a relatively "low cost" operation in a national church where parishes across the country raised a record £929 million to fund mission and ministry.

The diocese has enough reserves to keep it going until the end of 2015 and then it will run out of cash completely unless worshippers give more.

The 2012 figures represent a modest increase on £916 million in 2011 and show that after three years of deficits, parishes in general have successfully reduced their expenditure and encouraged more giving, to reach a break-even point in 2012.

The Diocese of Truro, however, is not matching the more optimistic national trends.

Bishop of St Germans Chris Goldsmith says on the diocesan website: "We are already a low cost operation. Across the Diocese of Truro, regular expenditure per electoral roll member is the lowest of all the dioceses in the C of E – £8.20 per week compared to the national average of £12.10.

"We may already be under-investing in our churches and ministries. If we continue to chase down declining income by reducing costs we risk an organisational death spiral from which it would be difficult to recover."

The people of Truro are "not as generous" as others in our giving to the local church, he warns.

Regular income per member is the lowest of all dioceses – £5.80 compared to the national average of £8.40.

"Even when we take into account the reality of low Cornish incomes, our proportion of giving is amongst the lowest six dioceses in the C of E – 1.7 per cent compared to the national diocesan average of 2.1," says Bishop Chris.

"Bringing ourselves up to the national average percentage of income given would increase our annual income by about £800,000."

The proportion of planned/tax efficient giving in the diocese is also the lowest in the Church – 56 per cent compared to a national average of 65 per cent. Truro's performance in this regard actually got worse from 2011 to 2012.

Bringing the diocese up to the national average percentage of gift-aided donations would increase its annual income by around £75,000.

Dr Mike Todd, chair of Truro's house of laity, spoke to the diocesan synod in May when he made it clear that the rising deficit on the operating budget "must be tackled very quickly."

He warned this week: "Our reserves will only carry us forward for another couple of years or so, including 2015, at the current level."

He added: "We cannot solve this problem centrally simply by further cost-cutting – as is evident from the national statistics, we are already the diocese with the lowest expenditure per member."

The diocese has launched a new process called the Deanery Challenge to allow deaneries, working with parishes, to submit comments by the end of September on what commitments they would be prepared to make in order to bridge the gap.

"There is no escaping the fact that we have a difficult task ahead, involving a serious look at how we fund the people and resources needed to support mission and ministry at the local level, as well as what personal commitments, in terms of planned giving and gift-aiding, will be needed to take us forward effectively and more confidently."