The spending watchdog has found it impossible to measure if more than £12 billion being spent on overseas aid is making any difference to the Government's key development goals.
Funding is being channelled more widely through Whitehall but no department is in charge of monitoring how well the money is being used overall, the National Audit Office (NAO) warned.
Some departments are struggling to spend their official development assistance (ODA) because they do not have the staff and systems in place to deal with the increase in money they are handling, but Treasury deadlines mean there is a risk the funding "might be rushed", it added.
About £12.1 billion was spent on aid in 2015 across 14 Government departments and the figure is expected to have hit £13.3 billion last year.
After laws were brought in committing the Government to spending 0.7% of national income on ODA, a new aid strategy was drawn up setting out the main objectives ministers hoped the money would achieve.
As well as strengthening global peace, security and governance, the Government wanted to enhance crisis resilience, promote global prosperity and tackle extreme poverty.
But progress can only be measured for the last aim, the NAO report said.
"With only one of the four of the UK Aid Strategy's objectives supported by measurable targets, it is not possible to assess progress in its implementation," it said.
The Department for International Development (DfID) has improved its management of aid money but other departments are facing "challenges", with five of the 11 looked at by the NAO spending more than half of their allocation in the last three months of 2016.
In its report, the NAO also set out the trends in aid spending, including figures released last year showing DfID gave £275 million to upper middle income countries and £1,105 million to lower middle income nations.
Sir Amyas Morse, head of the National Audit Office, said: "The Government has decided that departments and cross-government funds other than DfID should have responsibility for expenditure which makes up the 0.7% aid target.
"This means that meeting the target has become a more complex undertaking and the resulting gaps in accountability and responsibility require more effort to manage.
"HM Treasury and DfID, together with other relevant bodies, should now focus on developing ways to demonstrate the overall effectiveness and coherence of ODA expenditure."
Aid charities raised concerns about the findings and called for better accountability in the way money is being sent.
Oxfam's head of UK policy Richard Pyle said: "This report shows that too often other departments - to whom the Government is giving increasing volumes of aid - are falling short in this respect.
"No financial increases should be made by ministers to non-DfID departments, without action to improve the transparency and quality of aid they deliver."
Neil Thorns, director of advocacy at the aid agency Cafod, said: "Given that overseas development assistance is increasingly being spent outside DfID, yet the report notes a lack of clarity about which part of Government is responsible for monitoring overall effectiveness, it's imperative the Government ensures all aid spending is being held to the same standards of transparency and effectiveness as money spent by DfID."
Kat Sladden, UK director of the ONE Campaign, which tackles poverty, said: "The progress DfID has made in managing its budget is to be applauded - but more accountability is needed across Government."
A DfID spokesman said: "DfID is responsible for 74% of the Government's ODA spending. Other Government departments have direct responsibility for their share of the development budget and are accountable to Parliament and UK taxpayers for how they spend ODA.
"The International Development Secretary continuously reviews all DfID spending and stops programmes deemed not to be delivering value for money or which fail to meet international development objectives."