Before the euro's launch in January 1999, the Bank of England issued a 110-page plan - everything from settlement timetables to roadworks on the big day - to ensure a smooth introduction of the common currency in the world's largest financial centre.

The plan was among quarterly reports, complete with euro-themed cartoons by the Bank's resident artist, issued by the bank from 1996 to 2002 to iron out bumps as euro zone members abandoned their old currencies. Britain stayed out.

Fast forward to 2012 and banks and brokerages in London are quietly preparing for a more unpredictable but potentially more destabilising event - the possible break-up of the euro.

This time they do not have the luxury of such detailed and leisurely preparations as they seek to minimise the volatility and disruption to their business that could follow if a country left the euro zone or the whole bloc broke up.

Such moves would not only trigger deep economic and credit risks, the unprepared could face the nightmare of having to quote and trade euro-replacement currencies in the $4 trillion (2.6 trillion pounds) a day FX market.

Many of the industry's big FX banks, clearing houses and trading platforms say they are looking at ways to ensure their systems can quickly deal with any change in the composition in the euro, the world's most traded currency after the dollar.

Some institutions say they have been preparing for a possible break-up since mid-2010, when Greek default fears flared.

In a recent interview, HSBC said it had analysed its ability to trade new currencies as soon as possible after they were announced, but added it was not yet in full contingency mode.

It's not that we are expecting a break-up of the euro, but we need a plan to deal with adding new currencies and how to settle them, said Vincent Craignou, global head of FX and precious metals derivatives at the bank, ranked No. 6 by market share in Euromoney's closely watched FX survey.

Banks had years - and more money - to prepare for the euro's launch, but the 2007-2008 credit crisis has left many in a weaker position to deal with a break-up, which could happen suddenly to minimise damage to the value of assets issued by an exiting country.

It took many years for the euro to come together, David Rutter, chief executive of ICAP Electronic Broking, which operates the electronic trading platform EBS, recently told Reuters.

A large number of people in the industry believes that, if the euro comes apart, it will come apart more quickly.

FX participants acknowledge the difficulty of preparing for an event which could be kept secret until the last minute.

Industry experts say smaller institutions are underequipped to deal with the initial disruptions to trade that could result should a country leave the euro, while adding that most big banks' FX desks are expected to weather the transition.

If Greece, for example, left the euro, banks would need to update dealing systems to trade a new drachma against other currencies - from the dollar to the Swiss franc - on a spot and forward basis.

The process may be more complicated than reviving dormant trading pairs, such as drachma/dollar, as the old trading software might be incompatible with modern systems.

MITIGATING TRADE RISK

While it remains unclear what a euro dissolution would look like, the event would unleash market volatility and fluctuations in volumes if investors sought a rapid exit from the new currencies that would follow.

A spike up in euro volumes in May 2010, when the single currency sold off on growing speculation about a Greek debt default, was an eye-opener for Barclays Capital, as it highlighted the need to constantly improve its trading systems.

The volumes we saw then were manageable but it gave us an indication of where the market would go, said Mike Bagguley, global head of foreign exchange at the No. 2 FX bank.

The volumes we've seen (last) year have far exceeded those volumes. So it was worthwhile to start to prepare.

To ensure it can handle potentially volatile and unusually active trading, ICAP says it has conducted tests to see if its EBS system - the largest currency platform in the world -- can effectively quote and trade all 17 legacy euro zone currencies.

Because we've tested the currencies, we are ahead of the market, and the pairs we have tested, we can turn them on overnight, ICAP's Rutter said.

Many in the industry say a currency exit would likely take place over a two- or three-day weekend, which would be enough time for trading systems to prepare.

(Our) currency dealing systems are specifically designed so that we can add or remove currencies very easily and quickly, Thomson Reuters, which also operates an FX trading system, said in a statement.

(This version corrects David Rutter's job title in 11th paragraph)

(Additonal reporting by Jessica Mortimer, editing by Nigel Stephenson)