The amount of taxpayers' money being spent on emergency humanitarian assistance trebled between 2010-11 and 2014-15 to more than £1 billion, and the aid budget risks being strained by being drawn into protracted crises, a watchdog warned.
The National Audit Office found that since 2011 the Department for International Development (DfID) responded to more than 30 crises, including the Ebola outbreak and the aid response to the Syrian war, spending £1.29 billion of its £9.52 billion budget on humanitarian assistance in 2014-15.
The spending watchdog raised concerns about the department's ability to decide how to end funding after a crisis.
It " does not have a comprehensive set of criteria which underpin whether, and then when and how, to exit from crises".
"Of the 32 crises that the department has responded to since 2011, it has designed programmes to support continuing involvement in 21," the report said.
"On an individual basis the department's teams have plans for moving from a crisis response on to the next phase of its interventions. But the department does not have a view of how its involvement across all of its crises might impact on the availability of funding for other purposes."
NAO chief Sir Amyas Morse said: "The Department for International Development is choosing to take action in an increasing number of crises in complex and dynamic environments, and is spending over £1 billion a year in doing so.
"Interventions of this type can last for several years and our international experience suggests that it is 'easier to get in than get out'. It follows that this type of crisis action is likely to represent an increasing proportion of DfID's budget, assuming that interventions continue to be initiated at the same rate as they have been."
In its assessment of disaster response spending the report concluded that "securing value for money in this context is inherently challenging for the department".
"It is well positioned to identify and then respond quickly to sudden-onset crises, and has established longer-term interventions for the more stable protracted crises, making it well placed to achieve value for money in those cases.
"However, the department's management of its more fluid and protracted crises has yet to reach a similar level of maturity."
The report said that while the department has "forged good working relationships with other Government departments" the relief effort in Nepal involved spending £3 million on deploying military helicopters which were never allowed to enter the country, with the cost ultimately falling on the DfID budget.
The Government was unable to get the necessary clearances for the three Chinooks to land in Nepal and they were diverted to India where they played no part in the relief effort.
"The total marginal cost to the Ministry of Defence for its support was £3.9 million, of which £3 million was for the military helicopters," the NAO report noted.
"T he department's budget will be decreased by £3.9 million and the Ministry of Defence's increased by the same amount."
DfID completed 123 fraud investigations in 2014-15, with gross losses of £2.3 million uncovered, of which £1.6 million was recovered.
The report said: "The department recognises that the nature of some humanitarian assistance, such as commodities and cash transfers, as well as the reality of working in conflict zones, can increase the risk of fraud."
Some £16 million was spent on contract staff in the 12 months to August 2015 and they had been "invaluable" in supporting the response in Syria, Nepal, Yemen and Sierra Leone, the report said.
But it said c ontractors can be expensive when deployed for long periods, and may lack experience of working within government.
A DfID spokesman said: "The NAO is right to recognise the UK's strong track record in humanitarian response. From the Syria crisis to fighting Ebola, the UK has proved itself to be a world leader.
"As the number of complex and protracted crises continues to rise, our work in some of the most dangerous places is not only saving lives but also benefiting Britain by building greater stability and security overseas."
"As with all DfID spend, we apply the most rigorous checks to ensure our aid reaches those who need it and achieves the very best value for taxpayers' money."