Brexit is already leading to a “palpable decline” in British influence at the UN, and that influence would be in freefall but for the UK’s commitment to spend 0.7 % of gross national income on overseas aid, a study has found.

The report by the UK branch of the United Nations Association suggests Britain will lose political capital on the 15-member UN security council and the larger general assembly in New York because its campaigns will no longer be automatically aligned with those of the EU.

Based on more than 40 interviews, on and off the record, with current and previous UN-based diplomats, the report suggests the foreign secretary, Jeremy Hunt, will need to work harder to highlight a more value-based diplomacy, and a commitment to multilateralism, to retain the UK’s traction.

It says few overseas diplomats regard the phrase “Global Britain” as anything other than a slogan for domestic consumption.

The British Academy-funded report is one of many warning that the UK may lose diplomatic sway after Brexit, even though the UK’s permanent seat on the security council is not under challenge.

Two former UK ambassadors to the UN highlight the importance of the aid commitment as a multiplier of UK influence. Sir Mark Lyall Grant, UK ambassador to the UN in 2009-15, told the inquiry the foreign aid commitment demonstrated “evidence of real intent to be a global power”.

David Hannay, a cross-bench peer and ambassador to the UN in 1990-95, concurred that without this continued commitment the UK’s reputation “would be in sort of freefall territory”.

A third former ambassador to the UN, Sir Jeremy Greenstock, said of Brexit: “Most other people – almost without exception – think we’ve shot ourselves in the foot.”

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One New York diplomat told the inquiry: “There is a perception that the UK is doing less and championing fewer and less difficult agendas. Most of the difficult diplomacy in 2018 on Syria and Yemen at the UN security council was handled by Sweden and the Netherlands.”

Sir Simon Fraser, the former foreign office permanent secretary, said: “Fundamentally and structurally, I think our position, and our leverage in international institutions, will be weaker once we have left the European Union”.

When asked whether Brexit had impacted the UK’s capacity for influence, a senior source from a non-EU state said: “Yes, I can feel the UK’s weight in the council is dwindling, you can feel it. Definitely when Brexit happens things will change.”

An EU source at the UN said: “One of the risky things about Brexit is the inconsistency of the mood, and you know to be effective in the security council is this sort of mixture of being confident, consistent, reliable.”

Non-UK diplomats told the inquiry that Britain “was less willing to criticise China now, in recognition of how important China will be to the UK after Brexit. These problems for the UK are self-reinforcing because if they tread more gingerly, they appear to other members as if they are less powerful and less able to take a leadership role. This poses risks for the UK as they attempt to navigate their position in the world post-Brexit.”

The authors suggest the UK should adopt a more collaborative stance at the UN, for instance by sharing the so-called pen-holding role on key files with members of the general assembly.

Last year the UK was the penholder on 12 files including Libya, Myanmar and Yemen, and, although admired, Britain has been sometimes accused of not acting as an honest broker in that role, but instead using its influence to prevent difficult issues being aired.

The authors suggest the UK could gain notice by being more willing to respond to the call by the UN secretary general, António Guterres, for “a quantum leap” in collective action on peace operations. Such an offer would fit with Hunt’s promise to do more within the Foreign Office budget to prevent atrocities.

The report also says the Foreign Office will need to identify gaps in its diplomatic capacity at the general assembly since the UK will no longer able to rely on EU for burden-sharing and support in New York.

France will become the sole member of the EU on the security council, raising questions about whether it feels under pressure to represent the views, not just of France but those of Europe.

The lead author of the report, Dr Jess Gifkins from the University of Manchester, said: The research showed the impacts of Brexit go beyond the UK and the EU to the UN, where the UK’s reputation is tarnished and its capacity for influence is weakened.”

Prof Jason Ralph, from the University of Leeds, who led the research project, identified “widespread confusion about what Global Britain means, and doubts about the UK’s capacity to deliver the resources to back up its diplomatic leadership”.