Most UK Christian mission agencies will have to close by 2050, according to a senior practitioner and consultant.

In a blog post entitled Mission Agencies 2050, the former executive director of Wycliffe UK, Eddie Arthur, compared declining church attendance in the UK since 1970 with the sharp rise in the number of mission agencies coming into existence during the same period.

He concluded: "One might wish to argue with the exact figures, but the trends are clear; as church attendance declines, the number of mission agencies in the UK increases. There are more and more agencies seeking support from a shrinking constituency. This is not sustainable even in the short to mid-term, much less by 2050."

He added: "It isn't rocket science to suggest that the number of mission agencies based in the UK will decline precipitously over the next 35 years."

Arthur suggested that too many agencies were trying to do the same sort of thing, that they should refocus on supporting indigenous Christians and that they should prepare for radical change.

He told Christian Today: "Western mission agencies have been incredibly successful and have helped the Church spread around the world. However, much of their raison d'etre is no longer there.

"Most agencies will go by 2050. The demographics are against us."

He called for agencies to take a hard look at their futures, saying: "Many mission agencies should close. Lots of them do very similar things. Agencies do compete for resources – they all produce magazines and prayer guides. If two agencies don't combine, they might both be lost whereas if they do combine they might survive."

Arthur said agencies also needed to be prepared to re-focus their efforts where they were most needed: "We are currently seeing a huge amount of mission work in East Africa – where there are more Christians than there are in the UK – but who is going to the mega-cities of Asia?"

He acknowledged that change would be very painful, not least because of the personal emotional and financial investment many Christians made in supporting or working for mission organisations.

"The hardest thing in the Christian world is to stop doing something," he said. "People give their lives to a cause and live sacrificially – they don't have high salaries or company cars. To turn your back on that is very hard."

Arthur said that the long-term future of agencies was not widely discussed in mission circles. "A big concern is that those who have a degree of influence and can give a lead are board members, and they don't have time to grapple with the issue."

Most agencies based purely in the UK would be very small-scale, he said, though large organisations whose work was primarily in relief and development and carried out a large proportion of their work with government funding might survive. However, he warned of the risk to their specifically Christian character. "The danger with large organisations is that in order to get government contracts they will need to play down their Christian element. This is a big risk. The current government has been supportive of their Christian ethos, but I don't know whether this is a blip."

Arthur's research on the number of mission organisations started in the last 40 years was based on those affiliated to the Global Connections network, to which he is seconded from Wycliffe. However, he stressed that the rise was based on when the organisations were founded rather than a simple growth in the number affiliated to Global Connections.

In a previous blog post, Arthur warned of the risks to the UK Church posed by increasing secularisation and declining numbers. "While the Church won't be actively persecuted by the state, many of the privileges that we take for granted today will have been stripped away," he wrote. "'Promotion of religion'" will no longer be accepted as a charitable objective and many churches and Christian charities will no longer benefit from gift-aid or other tax-exempt donation schemes."

He also forecast that: "The decline in church numbers will place many Christian institutions at risk. It will simply not be possible for the current number of christian charities, seminaries, colleges and mission agencies ... to continue."