Carefully guarded in the Bank of England's vaults are a small number of very large banknotes. Called "giants" and "titans", they are not in circulation for good reason - each is worth a sum of money most of us can only dream of. What are they for?

"When it comes to a £1m note, everybody thinks, 'What a fantastic thing'," says Barnaby Faull, head of the banknote department at the auctioneers Spink.

"What most people don't realise is they do actually exist."

But the £1m pound note - known as a "giant" - is not in circulation and it is inconceivable it will be made available from cashpoints. How many of us would risk carrying one around in our wallet, let alone have sufficient funds in our account to get one out?

But even the monetary value of the giant is relatively small compared to the "titan" - a banknote that promises to pay its bearer £100m.

Impractical though they are for everyday use, both play a vital role in the British currency system, by backing the value of the everyday notes issued by commercial banks in Scotland and Northern Ireland.

Many people know how a Scottish fiver can be viewed with suspicion by businesses in England. This backing aims to maintain everyone's confidence in the value the notes represent.

For every pound an authorised Scottish or Northern Irish bank wants to print in the form of its own notes, it has to deposit the equivalent amount in sterling with the Bank of England.

If necessary, notes from, for example, a struggling Scottish bank could be replaced with regular Bank of England cash.

"If there was an unfortunate situation when one of the banks failed, note holders would have the confidence that their notes were still valued as it said on them," says Victoria Cleland, head of notes at the Bank of England.

Very occasionally, older £1m notes have escaped from the Bank of England's vaults and archives.

Faull recalls being offered a cancelled £1m note issued in connection with the Marshall Plan - the US's post-war aid programme to Britain. It had been presented to a retiring chief cashier and his widow later offered it for sale.

He says the Bank of England asked him not to publicise the sale. "Million pound notes were not supposed to be out in the open."

To read the complete article, see:Britain's £1m and £100m banknotes(www.bbc.co.uk/news/magazine-21145103)