UK shares opened lower on Monday, as multinational stocks tumbled on reports U.S. President Donald Trump privately discussed firing the head of the Federal Reserve and a partial U.S. government shutdown weighed on investor sentiment.

The FTSE 100 was down 0.8 percent and the mid-cap index was 0.5 percent lower by 0828 GMT. The bourses were due to close early ahead of Christmas.

Dragging down the UK's main index were companies that have a greater international presence, with Imperial Brands  dropping over 3 percent and British American Tobacco  1.8 percent lower.

Diageo, the world's biggest drinks marker, and consumer goods giant Unilever were down 1.6 percent and 0.7 percent, respectively.

All constituents in the consumer sector were in the red in early deals on the blue-chip index.

WPP, the world's largest advertising firm, fell nearly 3 percent to the bottom of the blue-chip index.

Most global markets were hit following reports that Trump discussed firing Fed chair Jerome Powell, a move that would likely further rile financial markets.

That added to pre-existing nerves over political stability in the world's largest economy after a partial federal government shutdown as Democrats rejected Trump's demand for more funds for a wall on the border with Mexico.

Among the UK mid-caps, gambling software company Playtech  slid 6.3 percent to its lowest in over six years after flagging that a change in Italian gambling tax law would hit its 2019 core earnings.

Oil heavyweights Shell and BP managed to eke out some gains as crude prices rose on signs the recent price plunge may start crimping supply from the United States.

Martin Sorrell's S4 Capital was flat on its first trading day on the London Stock Exchange's main market.