The gang behind the £22 million Northern Bank heist in Belfast may be forced to dump more than half the cash, it emerged last night.

Police chiefs hunting the robbers revealed at least £13 million of the missing money is made up in new notes.

Money laundering experts in Britain believe it will be virtually impossible to shift the cash without raising suspicions.

With police refusing to rule out republican or loyalist paramilitary involvement in the raid, Jeffrey Robinson, author of The Money Launderer, said: "If they are smart they will take the US dollars and euros and burn the rest."

Detectives revealed £12 million in new Northern Bank £10 and £20 notes and more than £1 million worth of £100 and £50 notes were among the stolen cash.

Detectives said considerable pre-planning was made in advance of the raid on a cash distribution centre in the basement of Northern Bank's headquarters.

Superintendent Andy Sproule, who is heading a team of 45 detectives, admitted: "This was a carefully planned operation by professional criminals who obviously had done their homework."

It has also been revealed the gang posed as police officers when they held the families of two bank employees hostage for more than 24 hours.

The centre's security is also under scrutiny.

Meanwhile, the bank admitted it had no external insurance cover and its Australian owners will have to bear the cost of the £22 million loss.

Detectives are trawling through hundreds of hours of CCTV footage.

They are examining tapes from cameras positioned in and around the headquarters of the bank.