The watchdog warned that nearly 5,000 houses or flats, and more than 10,000 childcare and school places, will be needed to honour the pledge.

The NAO said the total sum is uncertain as it depends in large part on the level of medial treatment refugees need and the extent to which they can gain employment and become self-sufficient.

Based on department assumptions, it estimated that the cost to the end of 2019/20 could be up to £1.1 billion, and up to £1.73 billion over the lifetime of the project.