* FTSE 100 down 0.3 percent
* Carnival hit as analysts react to profit warning
* Centrica, SSE drop on opposition price cap plan
* Tesco knocked by JPMorgan downgrade (Updates prices)
By Tricia Wright
LONDON, Sept 25 (Reuters) - Britain's top shares fell on Wednesday, led down by Carnival after a profit warning triggered downgrades, while Centrica and SSE fell after the opposition Labour Party talked of an energy price freeze.
Carnival slid 6.7 percent to 2,098.26 pence, the FTSE 100's top faller for the second session in a row, after Tuesday's warning of a possible loss prompted Morgan Stanley to downgrade the cruise operator to 'underweight'.
The stock has suffered its biggest two-day drop since mid-January 2012, down some 12 percent.
A JPMorgan downgrade knocked Tesco, off 3.5 percent, with the bank moving its rating on the grocer to "underweight", believing the UK food retailing industry to have structural problems and that "Tesco will be most impacted".
Other top fallers included Centrica and SSE, which were left nursing respective declines of 5.3 percent and 5.8 percent after opposition leader Ed Miliband said he would cap energy prices if elected in 2015.
Centrica was also trading without entitlement to the latest dividend payout on Wednesday, alongside Old Mutual and RSA, down 1.7 percent and 1.9 percent.
The FTSE 100 closed down 19.93 points, or 0.3 percent, at 6,551.53 points, with concern about the outlook for U.S. monetary and fiscal policy keeping investors on edge.
Expectations of reduced stimulus in the United States - which abated when the Federal Reserve left policy unchanged at last week's meeting, sparking a rally in equity markets - were reignited by New York Fed President William Dudley on Monday.
But some traders were encouraged by the fact the index is proving so resilient, down just 0.5 percent this week.
"I think there are significant reasons why we could have gone lower over the course of this week so the fact that we've just drifted off I think you can read as a positive," CMC Markets sales trader Matt Basi said.
"We're seeing very little money flow out of equities; our clients are generally positioned the same way that they were last week when markets (in Europe) were breaking new highs."
The U.S. economy faces a federal government shutdown if politicians cannot agree on a budget by the end of the month.
However, Commerzbank economist Peter Dixon cited valuations and the global growth backdrop as supportive of the British index.
"Valuations look okay. The international environment is clearly uncertain... (but) the global economic environment is beginning to if not significantly pick up then certainly brighten," he said.
The FTSE 100 trades on a 12-month forward price/earnings ratio of 12.2 times, against its 15-year average of 14.7 times, according to Thomson Reuters Datastream. (Editing by Christina Fincher)