Authorities haven’t released an exact figure of the amount stole – the bank saying only a “significant” amount of money had been taken.

After the families of two senior banks executives were taken hostage on Sunday, the thieves took two hours to clear out the underground vaults of the Australian-owned Northern bank in Belfast.

Both executives were “key holders” with privileged vault access.

With their families held at undisclosed locations, the men went to work on Monday morning with strict instructions to behave normally.

It was at the close of business on Monday that the operation to empty the cash vaults of the distribution centre was launched.

The hostages were released unharmed Monday evening.

The building at the centre of the robbery houses the bank’s cash centre, where tens of millions of pounds are believed to have been stored.

In one of the busiest shopping weeks in the run-up to Christmas, large amounts of cash would have come in from businesses in the city.

The centre also takes in, sorts and distributes newly printed notes.

Despite their initial success, the gang may have problems disposing of their haul.

Most of the money stolen was in Northern Bank notes, with the rest in sterling printed by other banks in the province.

Authorities believe this will make it very difficult to move in any sizeable quantities elsewhere in the UK or Irish Republic without raising some form of suspicion.

Northern Ireland’s elite crime operations team, whose officers specialise in investigating serious and organised crime in the province, is running the investigation.

Police were surprised by the size of the haul, and believe the gang may have had inside intelligence, in particular access to vital security codes.

Assistant Chief Constable Sam Kinkaid said a bank audit had yet to establish exactly how much was seized., but confirmed at least £20m had been stolen.

He also said it was “far too early” to say whether there was “any connections to a paramilitary group”.

Police officers took surveillance tapes from various cameras at the building, including ones outside the bank.

The size of the robbery put it on a par with the 1983 assault on the Brink’s Mat security warehouse near Heathrow when £26m of gold bullion was stolen by armed men posing as security guards.

Founded in 1824, the Northern Bank controls approximately 30% of the banking marking market in the province.

The bank describes itself on its website as “the largest retail bank in Northern Ireland with a network of 95 branches and 11 business banking centres”.

Last week, National Australia Bank announced it had reached an agreement to sell the Northern Bank to the to Denmark’s biggest bank, Danske.