Workers at human rights organisation Amnesty International in London will stage a 24-hour strike in disputes over job losses.
Unite said hundreds of its members at Amnesty International UK (AIUK), and at its global headquarters, will walk out in separate rows over issues including jobs and spending cuts.
Picket lines will be mounted outside Amnesty offices on Tuesday following industrial action earlier this year.
Unite said the two disputes involved separate management decisions which will result in job losses and which the union feared would undermine work done by the organisation.
Regional officer Alan Scott said: "Uniquely, these two separate disputes within the Amnesty family have converged and are linked by poor management decisions.
"Our highly dedicated members at both the AIUK and the International Secretariat are very reluctantly taking industrial action again. They want their respective managements to engage in a constructive dialogue to chart a fair and equitable way forward."
An Amnesty spokesman said: "At Amnesty UK industrial action has followed our embarking on a cost-savings programme to enable it to make a larger financial contribution to the global Amnesty movement, funding our Moving Closer to the Ground project.
"The union's ballot was over redundancies and not renegotiating this financial contribution.
"At the International Secretariat, the strike follows a dispute about essential changes in working practices. The reorganisation of the International Secretariat will see staff transfer from a centralised London base of more than 500 people to 10 regional hubs around the world, located closer to where human rights violations occur.
"Decisions over Moving Closer to the Ground have been taken, democratically, by our highest decision-making body, the International Council Meeting."