The armed gang behind yesterday's £40 million cash heist may discover that stealing the money was the easy bit.

Financial experts believe the robbers face a tricky task in spending the cash without being caught.

The euphoria of escaping with the huge haul is likely to have been replaced by a stark realisation of the difficulties it causes, according to specialist John Horan.

He said: "I don’t think they knew they would get this much cash. They planned for millions, but not this much. To start with they probably could not believe their luck, but now they’re getting the headaches."

Mr Horan, who works for accountants Harbinson Mulholland, believes strict anti-fraud regulations would make it extremely difficult to sneak much of the money into the British economy.

"The days of everybody taking cash are long gone. You can’t just walk into a bank, and businesses have to report transaction with large sums of cash."

The raiders will be helped because it is logistically impossible for the Bank of England to replace the entire currency, as happened following the 2004 Northern Bank robbery in Belfast, he said. Northern Bank responded to the theft by replacing all of the banknotes in circulation.

Used notes are also easier to launder than newly-minted cash, where serial numbers are readily identifiable.

The only real option amid a massive police operation is to take the money abroad, according to Mr Horan.

"It’s going to have to leave the UK. They will look for countries in a state of flux, or that have poor border controls. The Balkans would be a good place to start. If it were me I’d be setting up a front company in the Balkans and laundering it through that."

Mr Horan said the size of the robbery meant there was plenty of cash to offer as bribes.

"In countries where controls are lax, customs could give them a certificate saying the money had been explained and then the banks will take it without asking questions."

But the robbers were likely to stash the money and lie low for some days before trying to move it.

"The first 24 hours in any investigation is crucial. They will wait until the heat dies down and then start to shift it."

If the gang are unable to take the cash abroad they could use a technique known as "Smurfing", according to Mr Horan.

"A trick for money laundering is breaking it into smaller amounts and giving it to ‘Smurfs’ - lower level criminals. They get them to open up bank accounts.

"But the more Smurfs, the more people who know about it and the more chance of getting caught."

The robbers would be likely to contact organised crime syndicates to set up such a large Smurfing operation, giving them a significant cut of the proceeds, he added.

"They can expect to lose 10 to 15 per cent in the laundering process. It could be more."

Mr Horan said he was convinced the raid had not been carried out by beginners.

"These sort of robberies aren’t carried out by new kids on the block. To carry out a crime with this level of sophistication you need to have been around for a while."

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