The charities - which include Save the Children UK, Oxfam GB, and Christian Aid - have written to the prime minister and Treasury warning the decision to cut the UK aid budget from 0.5% of GDP to 0.3% would "destroy Labour's legacy on international development".
"As 138 leaders of the UK INGO sector, responding to urgent humanitarian emergencies and supporting global development, we are appalled by the recent announcement that UK aid will be cut to pay for defence spending," the letter read.
At the press conference, Sir Keir denied Mr Trump was effectively setting UK government policy, saying the defence spending increase was "very much my decision" and he had been "arguing for some time" that Europe and the UK "needed to do more".
The leaders of the non-governmental organisations, which also includes Action Aid UK and Islamic Relief, said they were "appalled" by the decision on the grounds it risked closing programmes that supported the most marginalised communities facing poverty, conflict and climate change.
Labour's decision to cut foreign aid follows a similar move in the US, where almost all workers are being pulled out of the US Agency for International Development (USAID) as part of Elon Musk's efficiency drive.
