Human rights workers at Amnesty International are braced for scores of redundancies after the management admitted to a hole in its budget of up to £17m to the end of 2020.

Up to 70 jobs will go in voluntary and compulsory layoffs amid a slump in donations from the national branches compared to forecasts last year, and a multi-million pound increase in spending on fundraising, the Guardian has learned. Staff have been told the organisation will be reshaped in line with the vision of recently appointed secretary general, Kumi Naidoo, who wants to increase Amnesty’s work on climate change and economic rights. There are concerns that cuts will marginalise in depth research on totemic Amnesty causes such as the death penalty, torture and the arms trade.

Next week Naidoo is due to unveil a new strategic direction, which he has previously indicated requires treating issues like climate change as core components of the human rights struggle. Sources said it appears he wants to increase the focus on campaigning, rather than traditional research-led human rights investigations.

Naidoo, who until 2015 ran Greenpeace International and was arrested for scaling an oil drilling rig off the coast of Greenland in 2011, has said it is time to “break away from old ideas that human rights are about some forms of injustice that people face, but not others”. He has spoken out in support of civil disobedience, and is a backer of the Extinction Rebellion and school climate strikes.

Managers have warned the secretariat’s 650 staff that “redeployment opportunities may be limited”. The Unite union, which represents staff, fears cuts will fall heaviest on lower income staff. It said that in 2017, the top 23 highest earners at Amnesty International were paid a total of £2.6m– an average of £113,000 per year. Unite is demanding a review of whether it is necessary to have so many managers in the organisation.

One insider said the 58-year old global organisation was in the grip of “an existential crisis”. The union resolution described it as “a perfect storm of challenges”.

Kumi Naidoo will unveil Amnesty’s new strategic direction next week. Photograph: Paul O'Driscoll

The job losses follow a damning report into the charity’s culture in February, commissioned after the suicides last year of two staff members, Gaëtan Mootoo and Rosalind McGregor. It detailed a “toxic” working environment and widespread bullying.

It warned: “As organisational rifts and evidence of nepotism and hypocrisy become public knowledge they will be used by government and other opponents of Amnesty’s work to undercut or dismiss Amnesty’s advocacy around the world, fundamentally jeopardising the organisation’s mission.”

According to a resolution from unionised staff in response to the threat of cuts “much of the anxiety experienced by staff in recent years has been generated precisely by the kind of mismanagement of finances and unfair treatment of staff that once again we see displayed in measures now proposed by the senior leadership team”.

The memo shows the charity was on course for a shortfall of £7m on its £20m current spending period, and that travel budgets have already been cut and a hiring freeze extended. The shortfall includes £2.5m spent on fundraising that it could not afford. Income from Amnesty branches around the world was £4.5m less than forecast.

Amnesty International said on Friday the decision to cut staff was “painful and difficult”.

“Even though membership of Amnesty is increasing worldwide, the International Secretariat will have to unfortunately cut its expenditure, while at the same time ensuring future priorities,” it said in statement.

“We are working closely with staff and the union to find a solution though we can confirm that there will be redundancies. Kumi Naidoo will in due course be sharing with staff details about the strategic direction of the International Secretariat, including his decision on changes to the leadership team, to ensure that it is in the best possible position to support the wider Amnesty movement going forward. We cannot provide further details until staff have been fully consulted.”

• This article was amended on 30 April 2019. Mention of “a slump in donations” has been changed because it is in comparison to forecast donations that receipts from national branches have dropped.