FTSE 100 shares posted solid gains on Wednesday as investors welcomed the US Federal Reserve's upbeat economic outlook, analysts said.

London's benchmark index gained 0.43 percent to finish at 5,644.63 points.

"The FTSE's early gains today were fuelled by yesterday's positive forecast from the US Federal Reserve, which came after UK markets had closed," said analyst Anthony Grech at financial spread-betting firm IG Index.

"The Fed pledged to keep rates in the current 0-0.25 percent range for an extended period of time, while simultaneously providing a generally upbeat assessment of the US economy.

"The mining sector was the UK market's engine room... as hopes for global growth were buoyed by the Fed's report."

Lloyds Banking Group (LBG) was the most traded stock, seeing 164 million units change hands, followed by Royal Bank of Scotland (RBS), which saw 110 million shares switch owners.

The day's top performer was software development group Autonomy Corporation, which added 79 pence -- or 4.55 percent -- to finish at 1,815, followed by silver miner Fresnillo, which added 28 pence -- or 3.39 percent -- to stand at 854.

Security services group G4S was the worst performer for the second day running, dropping 13.3 pence -- or 4.94 percent -- to close at 255.9, followed by insurer Standard Life, which shed 8.4 pence -- or 3.88 percent -- to finish at 208.3.

Meanwhile, sterling ticked up again against both the dollar and the euro.

At 17:10, the pound was trading at 1.5364 dollars, up from 1.5182 at the same time on Tuesday, while the currency stood at 1.1160 euros, up from 1.1040 over the same period.