* Barclays, JP Morgan, Citigroup, RBS and UBS targeted
* Claim estimated to be worth at least a billion pounds
* Banks have paid out $11bn in fines for FX rigging (Adds law firm comments, Citi and Barclays decline to comment, context)
By Kirstin Ridley and Iain Withers
LONDON, July 29 (Reuters) - Barclays, JP Morgan , RBS, UBS and Citigroup are being sued by investors over allegations they rigged the global foreign exchange market, in a test of U.S.-style class actions in Britain.
The claim, estimated to be worth more than 1 billion pounds ($1.24 billion), was filed at the Competition Appeal Tribunal (CAT) on Monday, U.S. law firm Scott + Scott said.
JP Morgan, RBS, UBS, Barclays and Citi declined to comment.
Some of the world's biggest investment banks have already paid more than a combined $11 billion in fines to settle U.S., British and European regulatory allegations that traders rigged the currency markets.
Litigators have long hoped to replicate in Britain the success of U.S. class action claims against banks, including Goldman Sachs, HSBC and Barclays, that have resulted $2.3 billion in settlements for big investors.
In May the European Union fined five banks a combined 1.07 billion euros ($1.19 billion) for forex rigging through cartels of traders known as "Essex Express" and "Three Way Banana Split".
The lawsuit is being led by Michael O'Higgins, the former chairman of British watchdog the Pensions Regulator, and is being funded by litigation finance group Therium.
O'Higgins told Reuters the total value of the claim would depend on the number of forex trades executed in London for UK-domiciled units - which will be automatically included in the action - and the proportional impact of rate rigging on these.
Given the size of London's forex market, O'Higgins said the total value would likely exceed a billion pounds.
"Even on a relatively conservative assumption it's certainly a billion pounds and possibly several," O'Higgins said.
"Markets should be fair as well as free and in this case the markets weren't fair."
The "massive" action is a "perfect" case to be brought as a so-called opt-out collective class action for breaches of UK or European Union competition law, David Scott told Reuters.
"It is a very difficult case to put together individual damages which are significant enough," the Scott + Scott lawyer added.
Britain's Consumer Rights Act (CRA) in 2015 introduced "opt-out" class actions for breaches of British or EU competition law. In such cases, UK-based members of a defined group will automatically be bound into a legal action unless they opt out, saving on hefty advertising costs. Overseas-based claimants, however, will still have to actively sign up.
The regime is designed to offer a more effective route to compensation for consumers and businesses who fall victim to anti-competitive conduct and is overseen by the CAT.
Its first major test case -- a 14 billion pound claim against Mastercard for allegedly overcharging more than 45 million people in Britain over a 16-year period -- was blocked by the CAT in 2017, a decision that was overturned at the Court of Appeal and is set to be heard by the Supreme Court.
This wrangling has already delayed other class actions and some law firms have chosen a different legal route for offering pension funds, asset managers and other institutional investors the chance to hold banks to account.
Law firm Quinn Emanuel Urquhart & Sullivan in December filed a damages claim against six banks through London's commercial courts, which it said has already signed up some of the biggest institutional investors. ($1 = 0.8049 pounds) ($1 = 0.8990 euros) (Reporting by Kirstin Ridley and Iain Withers Editing by Rachel Armstrong and Alexander Smith)