The Southern Baptist Convention is to cut between 600 and 800 jobs from its international mission board, representing 15 per cent of the total. The cuts are needed to account for a $21 million budget shortfall in the 170-year-old organisation, the board said in a statement.

Those who leave the mission will not be stepping "onto the sidelines of mission", but instead will be moving into a new phase of involvement in mission, the board said in a statement that outlined how the cuts fit with God's "sovereign" plan.

Over the past six years, the board has exceeded expenditure by $210 million in total. As many of the job cuts as possible will be achieved through methods such as voluntary retirement and fewer appointments. Global assets will also be sold off to keep missionaries in the field for as long as possible and plans to send hundreds of new missionaries out in the next two years will go ahead.

The cuts are announced after a period when the devoutly conservative organisation has campaigned tirelessly against same-sex marriage. The board has 4,800 missionaries and 450 other employees.

David Platt, board president, said: "Sure, this is not an ideal step but quite frankly there are no ideal steps at this point." He said continuing to allow outgoings to exceed income was not an option. "For the sake of short-term financial responsibility and long-term organizational stability we must act."

Staff were told of the plan at a town hall meeting yesterday. Trustees were told earlier this week.

Platt said: "We praise God for the reserves and property sales that made this possible and for leadership which chose to spend these resources for the spread of the Gospel."

Sebastian Traeger, vice president of the board, said several options had been considered.

"The challenge is that we're looking at both large revenue shortfalls and low cash reserves – so any action needs to include a plan to address both simultaneously. We considered multiple options, such as further reducing missionary appointments or liquidating additional property, but none of them bring about a balanced budget fast enough, or they are not feasible to implement in the short term. Our goal is to align our cost structure with the amount of money given to us each year."

In a Q&A posted on its website, the board said it had been able to cover costs through reserves and property sales in the past but this was not a long-term solution. It does not have an endless supply of properties, it explained, and there are many complexities involved with selling overseas property and repatriating the funds. Moreover, the board is now close to depleting its reserves.

In 2009 the board hit a high mark of 5,600 missionaries on the field. Since then, missionary numbers have decreased to their current level. The number needs to go down further to 4,200.

The cost of personnel is about 80 per cent of the total budget.

The board said that while the issue at hand is financial, it is ultimately spiritual.

"God is not surprised by these financial realities. He has reigned sovereign over the IMB for 170 years, and He will continue to reign sovereign over the IMB for years to come. God has reigned sovereign over the direction of each personnel's life to this point, and He will reign sovereign over these lives in the days to come.

"Because He is sovereign, IMB leadership encourages all of its personnel to seek Him, and ask Him how and where He is guiding each of them for the sake of His name. IMB leadership believes that, without question, God will continue to lead every one of its personnel on mission."