UPDATE 2-Tullett revenue rise dulled by U.S. regulatory move

* Jan-Apr rev up 10 pct, down 6 pct at constant currency

* U.S. unveils plan to force central clearing for CDS

* Structured products, emerging markets volumes reduced

* Shares down as much as 8.5 pct

(Adds comments, details, updates shares)

By Daisy Ku

LONDON, May 14 (Reuters) - British interdealer broker Tullett Prebon's (TLPR.L) announcement of a 10 percent rise in four-month revenue on Thursday was overshadowed by a U.S. plan to increase derivative regulations and lower trading volumes.

The Obama administration on Wednesday unveiled a plan to force all standardised over-the-counter (OTC) derivatives to be cleared through a central platform, an attempt to increase transparency and reduce risk in a market that has been largely unregulated. [ID:nN13414280]

The plan would have a negative impact on interdealer brokers as trading in derivatives, such as the credit default swap (CDS) market, could move more easily onto exchanges.

"Our long-term concerns remain over Tulletts' competitive position given its lacking electronic broking or post trade capabilities," said Vivek Raja, an analyst at Panmure Gordon. "Particularly in light of the push towards product standardisation in the OTC space."

If market uncertainty continues, it could also limit Tullett's ability to reallocate resources and further cut costs in response to sudden market shifts, Raja said.

Tullett, the world's second biggest broker between banks, said on Thursday that it had generated 354 million pounds ($537 million) in revenue in the first four months of the year, up 10 percent from a year ago, thanks to a favourable currency impact.

At constant exchange rates, revenue dropped 6 percent, pointing to a fall in trading volumes.

Analysts expect Tullett to continue to see lower trading volumes and margin pressure over the course of the year.

Tullett shares fell as much as 8.5 percent and by 1022 GMT, they were down 6.7 percent at 295.8 pence, valuing the company at 8.3 times 2009 earnings per share estimates of 34.81 pence each -- a 29 percent discount to bigger rival ICAP (IAP.L). The pullback comes after a rally of 118 percent in the shares this year.

ICAP shares were down 1.1 percent at 374.3 pence.