The UK spending watchdog on Friday said post-Brexit border operations with the EU had been “largely successful” but warned temporary fixes made it unsustainable in the long term.

The National Audit Office looked into the arrangements since Britain left the European Union in full in January, which had stoked fears of long queues and bureaucratic chaos.

That mass disruption has not materialised was a “significant achievement”, it said, but warned the current system “cannot go on indefinitely”, as new checks loom.

Tighter controls at the frontier, including in busy Channel ports on the English south coast, are due to come into effect in the first half of 2022.

“The current overall operating model for the EU-GB-border is not sustainable, and much more work is needed to put in place a stable operating model that eliminates any risk of WTO challenge from trading partners, does not require any temporary supports, and has clarity and ease of use for border users,” it added.

The end to freedom of movement across the bloc and seemless trade within the European single market and customs union threw up a particular challenge in Northern Ireland.

The British-run province has the UK’s only land border with the bloc.

Complicating arrangements is the need to keep the border with Ireland open — a key plank of the 1998 peace deal that ended decades of violence over British rule.

A separate mechanism to the last-gasp trade deal signed on Christmas Eve last year was agreed for Northern Ireland.

But the so-called protocol, which effectively keeps Northern Ireland subject to some EU rules, has been met with opposition from pro-UK unionist groups in the province.

They claim checks on goods travelling from mainland Great Britain — England, Scotland and Wales — to prevent unchecked items entering the single market via Ireland threatens Northern Ireland’s position in the wider UK.

The government in London says it is unworkable in its current form and has threatened to suspend the protocol, even after the EU proposed a raft of concessions to assuage fears it threatened UK sovereignty.

The NAO acknowledged the situation in Northern Ireland was “inherently challenging” and said the government should strike a deal with Brussels on any amendments as soon as possible.

It said government help to facilitate trade had been “of value” but it would never have met an initial October 1 deadline to put in place everything required.

“It is critical that UK government departments and devolved Northern Ireland departments deliver quickly on any agreement reached between the UK and EU and put in place the systems, infrastructure and resources required to make it work,” the watchdog conclude.