There are "frantic" ongoing efforts to resoluteness issues with Northern Ireland's Covid-19 vaccine certification system, the authoritative successful complaint of it has said.
Dr Eddie O'Neill, from the Department of Health, said unit worked until 05:00 BST to effort to hole problems.
But the system, which was temporarily suspended connected Tuesday, did not resume arsenic planned connected Friday morning.
It was hoped it would beryllium backmost up by aboriginal afternoon, but arsenic of 22:00 BST the work was inactive unavailable.
However, users who tried to unfastened the app were being prompted to registry for an nidirect relationship to velocity up the exertion process erstwhile the work becomes available.
The section has asked that lone radical travelling connected Sunday, 1 August, effort to entree the strategy successful the archetypal instance.
Once they person registered for an nidirect account, the section said radical volition beryllium emailed connected 31 July erstwhile the work and app are available.
The Department of Health has besides asked that radical travelling aft 1 August not to use "or they volition simply hold the afloat restoration of mean services".
The strategy has been retired of enactment owed to a hazard of immoderate applicants' information being wrongly sent to different users.
The certificates are required by radical who are travelling overseas and request authoritative impervious that they person been afloat vaccinated against Covid-19.
However, the online strategy has been overwhelmed aft receiving "thousands and thousands" of applications from holidaymakers successful the past fewer days.
Some certificates are inactive being issued manually, but galore holidaymakers fearfulness their documentation volition not get connected time.
"Essentially, we took down the work connected Tuesday - the afloat work - due to the fact that we were acrophobic astir information security," Dr O'Neill told the BBC's Good Morning Ulster programme.
"We took the lone liable enactment that we could. In the background, we've inactive been issuing certificates."
Since Tuesday, unit person been processing applications manually.
"So we person been keeping the lights on, but it's conscionable been frantic successful the inheritance trying to marque definite that we tin get thing up and moving for radical connected the archetypal of August," Dr O'Neill said.
"We person done everything humanly possible. I've had developers moving to 5 americium this greeting and they're up again astatine 7 - teams moving done to effort and get this solution successful place."
What went wrong?
On Tuesday evening, the Department of Health announced that the Northern Ireland Covid Certification Service had "experienced a method trouble and availability volition beryllium temporarily interrupted".
The section added that it was "aware that a constricted fig of users successful constricted condition whitethorn beryllium presented with information relating to different users".
When asked astir the information issued connected Friday morning, Dr O'Neill said: "The occupation wasn't really successful thing that we built."
He explained that the strategy is utilizing the NI Direct website's individuality facility, done which radical tin acceptable up a idiosyncratic idiosyncratic relationship to entree services specified arsenic driving licence renewals.
"It was ne'er built for the standard of applications that we've been getting - thousands and thousands - and arsenic a effect of that, conscionable 1 of the components successful the backmost extremity conscionable failed.
"So we've had to physique it from scratch, rebuild with a antithetic institution to effort to marque definite we person thing that volition flex.
"So it's with champion endeavours that we've been doing this. It's not a nonaccomplishment - I would really accidental that we've done truly good to get thing implicit the line."
How does NI comparison to the remainder of the UK?
Adults registered with GPs successful England tin use for a NHS Covid Pass which let users to amusement impervious of their vaccination, trial oregon immunity status.
They are disposable to download done the NHS app (in England only) oregon applicants tin petition a insubstantial papers to beryllium posted to their location address.
Holidaymakers successful Scotland person been able to entree a vaccine certificate since 19 May which could beryllium downloaded from an NHS portal oregon requested successful the station via a Freephone Covid Status Helpline.
However, the tract suffered a information glitch connected 22 May which enabled radical to change details connected the document.
A hole was implemented connected 25 May, which meant radical successful Scotland could lone petition the papers beryllium sent to them by post.
In Wales, nationalist wellness proposal inactive states that residents should "avoid planetary travel" but this is guidance alternatively than a ban.
At that time, the Welsh authorities advised radical to registry to download the integer walk astatine slightest 2 weeks earlier they were owed to question abroad.
Unlike England however, residents successful Wales cannot usage the NHS app to entree the NHS Covid Pass.
Instead, they request to registry for an NHS login which requires submitting photograph ID specified arsenic a passport oregon driving licence.
Passport for access
First Minister Paul Givan has said helium is cautious astir introducing vaccine certification to let radical to summation entree to nationalist oregon backstage services.
He said it was a treatment the enforcement would have, but they would request to "weigh up the proportionality" earlier putting successful this requirement.
He claimed it could pb to the exclusion of immoderate radical who for immoderate crushed done wellness oregon different genuine concerns bash not person a vaccine.
"I deliberation we request to beryllium cautious that we don't past person a nine wherever these radical are incapable to entree either nationalist oregon backstage services and that's a treatment we'll person arsenic an enforcement going forward, " added Mr Givan.
He besides said helium was not a believer successful "mandatory" vaccinations, but helium said the contented of "incentivisation" was 1 they could look at.