The move "reflects our view that Deutsche Börse's financial profile would deteriorate following a merger with the more leveraged and lower-rated NYSE Euronext," S&P analyst Thierry Grunspan said in a statement.

For now, however, Deutsche Boerse's long-term credit rating remained at "AA" and its short-term debt still has the highest S&P rating of "A-1+".

On Tuesday, Deutsche Boerse posted a 2010 net profit of 417.8 million euros ($564 million), a decline of 16 percent from 2009 owing in part to fourth quarter charges related to its US stock-option trading unit ISE.

The German market operator's adjusted core earnings gained 5.0 percent to 1.1 billion euros on sales up 2.0 percent at 2.1 billion euros.

S&P estimated that a merger of Deutsche Boerse and NYSE Euronext, which would create the world's biggest stock exchange operator, will be better for the latter and placed it on a positive CreditWatch.

NYSE Euronext's ratings were confirmed meanwhile at "A+" for the long term and "A-1" for the short term.

Deutsche Boerse and NYSE Euronext signed Tuesday an agreement to create a massive market operator that is diversified and has a wide geographical reach.

It is expected to cover 40 percent of the US market for stock options trading and be the largest derivatives exchange worldwide.

"The transaction is likely to come under intense scrutiny from European antitrust regulators," S&P commented.

Deutsche Boerse and NYSE Euronext have set a year-end target to finalise the deal.