There are still "significant" risks attached to the smooth operation of the UK border as the clock ticks down to a potential no-deal Brexit, the UK's spending watchdog has warned.

With just two weeks until the UK's scheduled withdrawal from the European Union on October 31, the National Audit Office said that mitigating these risks is now, to some extent, out of the UK government's control.

The most significant risks to the operation of the border remain business readiness, EU member states imposing controls, and arrangements for the Northern Ireland border. 

In its report, the NAO said the UK government has made progress with putting in place the systems, infrastructure and resources required to manage the border if the UK leaves the EU without a deal on October 31.

But it said "there is still some work to do to finalise arrangements in the short time that remains and bringing all these elements together for the first time in a live environment carries inherent risk".

"It is impossible to know exactly what would happen at the border in the event of no-deal on October 31 2019. Departments face new challenges in monitoring and responding to any disruption that may ensue," it added.

"This includes supporting businesses and individuals in meeting their new obligations, mitigating risks of the border becoming vulnerable to fraud, smuggling or other criminal activity, and activating civil contingency plans if necessary," it stated.

The report says there are 150,000 to 250,000 traders, estimated by HM Revenue & Customs, who would need to make a declaration for the first time in the event of no-deal.

The NAO said the UK government's reasonable worst-case planning assumptions state that the flow of goods across the short Channel crossings could initially be reduced to 45-65%, taking up to 12 months to flow normally.

In forming this estimate it assumed that 30-60% of hauliers travelling to the EU border will have appropriate documentation. 

The watchdog found that despite efforts across the government, a "large proportion" of traders and businesses would not be ready for new customs and regulatory controls if the UK leaves without a deal and might not be able to access the support they require.

"Despite the Government's actions, it has been unable to mitigate the most significant risks to the effective functioning of the UK border in the event of no-deal and the border would be 'less than optimal'," the report said.

The NAO said the UK government has announced temporary arrangements for managing trade crossing the land border from Ireland to Northern Ireland, but it acknowledges these are "not likely to be sustainable", adding that there is still uncertainty about border arrangements that the Irish Government would introduce.

"Preparing the UK border for EU exit with or without a deal is extremely complex and has required a huge amount of work from many government departments, agencies and third parties such as traders," commented Gareth Davies, the head of the NAO.

"Despite their efforts, significant risks remain which may have consequences for the public and businesses.

"Government will face new challenges in monitoring and responding to any disruption that may ensue following a no-deal exit, and will need to replace temporary measures with sustainable long-term solutions to ensure the border is fit for purpose," he added.