Northern Ireland's civil servants had "limited visibility" of critical information from Whitehall as the original Brexit date approached.

That's according to an internal document released to BBC News NI under Freedom of Information rules after an appeal to the Information Commissioner.

Officials had initially resisted disclosure saying it could "weaken or inhibit" NI's negotiating position.

The document was circulated internally in June 2018.

Media playback is unsupported on your device Media caption What will become of the Irish border when the UK leaves the European Union?

It was a review of preparedness by the Department of Agriculture, Environment and Rural Affairs (DAERA), nine months before the UK's original date for leaving the EU in March 2019.

DAERA will have a key role to play after Brexit, issuing animal and plant health certificates and carrying out inspections that will be crucial to continued cross-border trade.

The so-called "Brexit Gateway Review" was carried out by experts in the Department of Finance.

It involved reviewing documentation and interviewing senior staff, including the permanent secretary Denis McMahon.

Its purpose was to assess whether DAERA's Brexit preparations were "appropriately focused and supported by the right governance and capacity."

'Must haves'

The review found that what was needed was a "clear tactical and operational focus" on immediate steps to prepare for Day 1 departure.

It found that DAERA's equivalent department in Whitehall, DEFRA, had identified 64 Day 1 projects of which up to 11 had been identified as "must haves" for exit.

It suggested that DAERA needed to identify any potential gaps and address them.

Image copyright PA Image caption The UK was originally set to the leave the EU in March 2019

But communication between Whitehall and Belfast appears to have been less than satisfactory.

"Northern Ireland officials have limited visibility of Brexit information even in areas that are critical to Northern Ireland interests," the review said.

The absence of a functioning assembly, executive and ministers to take decisions and represent Northern Ireland's interests was an added complication at what was a time of "very significant challenge", it said.

A spokesman from DAERA said it had "an excellent working relationship" with DEFRA.

'Chilling effect'

A section of the review which went into greater detail about the level of engagement between DAERA and DEFRA was redacted under a Freedom of Information (FOI) exemption before the report was released.

The review said as the original Brexit date got closer, communication with DEFRA had improved.

BBC News NI requested a copy of the document under FOI legislation in November 2018.

DAERA resisted releasing the review document as late as May 2019.

It said disclosure would "impair the integrity and viability of the decision-making process to a significant or substantial degree without a compensating benefit to the public.

"The chilling effect argument is high in this case as the Brexit situation was and still is live".

It also said it would damage the "safe space" needed to debate live policy issues "freely and frankly".

It finally agreed to disclose the document following an appeal by the BBC to Information Commissioner.

A spokesman from DAERA said: "We have an excellent working relationship with our colleagues in Defra, which has been further enhanced and strengthened as a result of the ongoing work around Brexit in the last year.

"We very much value the benefits of this collaborative approach and we will continue to actively work with Defra across a range of issues."