Christian Aid is to make at least 70 redundancies in a bid to save £6m over the next two years.
Christian Aid, which employs about 750 people and had an annual income of £83m in 2008/09, said a combination of a fall in donations and the weak value of the pound, which has hit the charity's international programme, had prompted the move.
"The biggest single factor was a £3m shortfall of hoped-for income during Christian Aid week in May," said John Davison, head of media at Christian Aid.
Davison said the charity aimed to save £3m this year and at least £3m next year.
Christian Aid is negotiating with trade union Unite and a formal consultation with staff will begin next week.
The total number of redundancies is not known but cuts will be made across all departments, said Davison.
Cafod also confirmed it will announce job cuts to staff this week but declined to say how many.
A spokesman said: "At the moment the managing-down process is still being finalised, so given the sensitive nature of the issue, we're not able to give any exact figures until after this week once all staff have been fully informed of what's happening."
A spokeswoman for Oxfam said the charity was "looking into a restructure next year".
She said this might include some job losses but was unable to confirm how many that could be.