* Q1 sales up 18 percent at constant currencies to 85.5 million sterling
* Growth likely to slow in current quarter
* Operating margins likely peaked in 2010
* Shares down 3.6 percent
(Adds shares; CFO, analyst comments)
By Georgina Prodhan
LONDON, Jan 20 (Reuters) - British financial media group Euromoney reported an 18 percent rise in first-quarter revenue at constant currencies, driven by strong subscriptions to its publications amid a bounceback in the financial sector.
Euromoney, the first of Europe's financial publishers to report this season, said comparisons with last year would be tougher going forward, with growth likely to slow in the current quarter.
It said increased investment in technology and new products, along with higher headcount costs, meant its operating margins had likely peaked in 2010 and would fall by about two percentage points this fiscal year to end-September.
Euromoney, which is majority owned by Daily Mail & General Trust, publishes magazines, journals and newsletters including Institutional Investor and Metal Bulletin, and provides training and data.
Euromoney shares fell 3.6 percent to 705.5 pence by 1208 GMT, underperforming a 0.6 percent-weaker European telecoms index, despite the better-than-expected sales of 85.5 million pounds ($136.6 million) reported on Thursday.
Analysts said they had bargained on a smaller drop in the operating margin, but London brokerages Numis and Collins Stewart both raised their target price -- to 884p and 815p respectively -- on the results for the October-December quarter.
Euromoney said uncertainty remained over the global economic outlook, sovereign debt in Europe, and whether China would tighten policies to reduce inflationary pressures.
Chief Financial Officer Colin Jones said the company expected to invest up to 10 million pounds in products and technology this year, up from 6-8 million pounds last year.
He said the company had added more than 250 staff from a low point last year, taking the total to just over 2,000 -- still less than its peak. ($1=.6257 Pound) (Editing by David Hulmes; Editing by Jon Loades-Carter)