The Government cannot fully explain what happens to taxpayers' money used to respond to humanitarian crises, according to an influential committee of MPs.
The Commons spending watchdog found the Department for International Development (DfID) often did not have a "full understanding" of where its money went when it used UN agencies and other partner organisations to deliver aid.
The Public Accounts Committee found that spending in response to humanitarian crises had trebled to almost £1.3 billion in the four years to 2014-15 and many aspects of the approach were "working well".
But the MPs said the department could "improve value for money, particularly in its response to those complex long-running crises, such as Syria and Yemen" and needed a better understanding of what constitutes a successful intervention.
The committee said the DfID needed to improve the oversight of partner organisations, such as UN agencies, and should push for reform of the international humanitarian system at a global conference in May.
"The value for money for the UK taxpayer of the department's funding of UN agencies is undermined by the overlapping remits of the agencies and inflexibility in their systems," the report said.
"UN organisations received around half of the department's £1,288 million spending on humanitarian activities in 2014-15.
"Despite being a major and well-regarded donor, the department has found it difficult to influence the UN to adapt its structure and the practices of individual UN bodies to improve their effectiveness."
The report added: "For many of its crisis interventions, the department does not have a full understanding of how much of the taxpayer's pound is spent by which bodies and on what."
The MPs acknowledged that partner organisations incurred a range of expenses - such as staff, food, supplies, transport and security - in delivering aid but DfID did not have a "good understanding of the size of the different cost components", making it hard to assess the best-performing bodies.
"Furthermore, in complex projects the department does not always know the range of organisations its first-tier partners are funding, making it difficult for the department to manage risks."
The MPs said the DfID accepted it "ought to be able to track" who is receiving and spending money from the UK taxpayer.
Committee chairwoman Meg Hillier said: "It is important to recognise the Department for International Development has had a positive impact in responding to some desperate situations around the world.
"It is also important that, however public money is spent, the Government is held to account for that spending.
"In responding to crises, achieving value for money means critical support can be provided more effectively to more of the people who need it. The taxpayer pound goes further.
"The department operates in difficult environments and the challenges it faces can change quickly. It is vital it looks constantly at the way it spends public money and ensures it is being used to best advantage.
"To achieve this will require work both inside the department and beyond. In particular, the department is an influential player globally and it must make full use of this influence to help secure change in the humanitarian system."
A DfID spokeswoman said: "We welcome the committee's recognition of the success DfID has had in tackling a range of crises, working in some of the most difficult and dangerous places in the world.
"The Government's aid strategy places a heavy focus on ensuring value for money, and the UK is playing a lead role in discussions on the future of the international humanitarian system and overseas development, including the role of UN agencies."