The government is struggling to spend its overseas aid budget because it doesn't have the staff and systems in place to deal with the increased in money it is handling.

The spending watchdog warned Treasury deadlines mean there is a risk the funding "might be rushed".

The National Audit Office (NAO) also said that it has become impossible to measure if more than £12 billion 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 NAO said.

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.

A new aid strategy was drawn up setting out the main objectives ministers hoped the money would achieve after laws were brought in committing the government to spending 0.7% of national income on official development assistance (ODA).

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.

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.

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."

A Department for International Development (DfID) spokesman said: