The UK government's aid programme in Libya could be inadvertently harming migrants and refugees, the Independent Commission for Aid Impact (ICAD) warned on 10 March.
The watchdog said some of Britain's £9m package is being used by the Libyan authorities to contain more people taken from the Mediterranean Sea in detention centres, where they face overcrowded conditions and "are at risk of abuse and extortion".
"According to the 'do no harm' principle, the responsible departments must take measures to minimise risks and ensure that residual risks are carefully monitored," ICAD said.
"This involves more investment in contextual analysis, in conflict assessment and in understanding the complex political economies in operation along the central Mediterranean migration routes.
"When engaged in particularly high-risk interventions, such as support to the Libyan coastguard or detention centres, aid-spending agencies should be explicit about the assumptions underlying their decision to engage, keep them under constant review and be willing to withdraw support if they no longer apply."
Libya is currently home to up one million migrants and refugees. But the country is not party to the 1951 Refugee Convention or its 1967 Protocol.
"We call for concerted efforts to ensure that sustainable migration and asylum systems are put in place in Libya, when the security and political situation permits, and in neighbouring countries," the UN Refugee Agency said in February.
"We urge a move away from migration management based on the automatic detention of refugees and migrants in inhumane conditions in Libya towards the creation of proper reception services.
"Open reception centres should offer safe and dignified conditions, including for children and victims of trafficking, and respect key protection safeguards."
A UK government spokesperson said: "Cross government efforts are tackling the root causes of migration by building opportunity and stability for people in their home regions so they don't need to make the perilous journey across the Mediterranean.
"ICAI rightly praises our innovative work in Ethiopia where we are creating 30,000 jobs for refugees.
"We're also getting help to vulnerable migrants who have already started their treacherous journey. Since May 2015, British vessels have saved more than 13,000 lives in the Mediterranean."