The Church of Scotland's 'very existence' is under threat with whole generations missing from its pews.

A major 10-year strategic review being discussed by the ruling general assembly in Edinburgh this week, recommends drastic restructuring of the 458-year-old church amid dire warnings about its future.

Membership has fallen by around 20 per cent in five years, from 413,000 in 2011 to 336,000 at the end of 2017. In the next five or six years there will be a shortfall of around 300 paid ministers with 75 per cent currently over the age of 50, the report says.

'There are missing generations in congregations,' it says in a stark assessment of the Kirk's future. 'Not only do we have very few children compared to 10 years ago, or similarly young people (under 25s), the number of folks in their 30s and 40s is also very small.

'Given that the majority of those attending Church are over 60, these missing generations pose a real challenge to the very existence of the Church. We cannot afford to ignore this and fail to produce a plan to address it.'

The Church of Scotland is the country's national church and operates on a parish system similar to that in the Church of England. However the report says 'the present model of one Minister to one parish is no longer sustainable' with the impending shortage both of ministers and funds to pay them.

The rapid decline means the Church is running at a deficit and has 'more buildings than it needs or can sustain'.

'It is obvious that falling congregational numbers and ministers will lead to a parallel fall in income. This means we will need to manage limited resources carefully and prioritise what we do,' it says.

A major challenge is the Church's social action arm, CrossReach, which employs 2,000 of the 3,000 staff who work for the Kirk.

By 2021 a 'Workforce Plan' will have been drawn up and implemented, the review says, 'ensuring appropriate levels of staffing to meet the strategic priorities of the Church'.

Asked whether this would mean job cuts, the Church's chief executive and the report's author Rev Dr Martin Scott, said: 'It's difficult to imagine, with the numbers shrinking, that we will need the same number of staff in 10 years' time. But I don't want to alarm people as no decisions have been made. We have to see what the response is to the plan then we'll work out from there what the changes are.'

Scott, who serves as secretary to the Council of Assembly, told the Sunday Herald: 'We can say if we continue at this rate we will not exist in 30 or 40 years, but that's not the story of the church. We've had crisis in the past and it's about how we turn this around.

'Maybe crisis is not the right word. It's a time of challenge, but it's also a time of opportunity. What we're trying to do is see if there's an opportunity for change. If we do nothing about it, it will become a crisis. That's what we're trying to avoid. We hope this strategic plan will give a sense of focus.'

By 2020 the Church of Scotland will begin culling churches 'where congregations have no long term future' the report says, as well as looking for 'appropriate places for new and emerging congregations'.

On the issue of buildings, the review says by 2028 each area will have a plan for the 'use, development or disposal of all buildings within their bounds'.

It says: 'We will ensure that an appropriate number of well-equipped spaces for worship, discipleship activities, service provision and engagement with the community are available. This will mean fewer buildings, but ones which are equipped and ready to sustain God's mission locally and nationally.'

As well as the challenges of dwindling congregations, falling revenue, crumbling buildings and ageing ministers, the Church of Scotland is also trying to grapple with its stance on same-sex marriage.

On Saturday the general assembly voted in favour of a motion that will take it a step closer to approving ministers conducting same-sex marriages. A motion to instruct its legal questions committee to prepare legislation with safeguards passed by 345 to 170. But commissioners agreed that the committee should only act if, in its opinion, these safeguards 'sufficiently protect against the risks they identify'.

The committee will report its findings to the general assembly in 2020.