LONDON (Reuters) - U.S. financial software firm SS&C Technologies has ruled out a counter-bid for Britain's Fidessa, which has agreed a 1.5 billion pound takeover by Ion.

Fidessa disclosed this month that it had received approaches from both SS&C and Ion, a company backed by U.S. private equity firm Carlyle. Fidessa had already agreed to be bought by Swiss banking software business Temenos but scrapped the deal on April 20 in favour of a better offer from Ion.

SS&C said on Tuesday that it held preliminary talks with London-listed Fidessa this month but had decided against making an offer for the trading systems provider.

It joins Temenos in ruling itself out after the Swiss company said last week that it would not try to better the Ion deal, which values Fidessa at 39.50 pounds a share, including payment of dividends due to investors in June.

Fidessa shares were down 2 percent at 39.30 pounds in afternoon trading in London. SS&C was up 1 percent.

(Reporting by Ben Martin; Editing by David Goodman)