A photograph of Warburton in full kit alongside coach Warren Gatland and his assistants at the west London hotel where the squad will be announced later on Wednesday (1100 GMT) appeared on social media.
Although the announcement is yet to be made official, Brett Gosper, CEO of World Rugby, earlier tweeted his congratulations to Warburton for becoming the second man, after England's Martin Johnson, to lead the Lions twice.
Warburton, 28, led the Lions in the first two tests in Australia four years ago but missed the decider through injury, when compatriot Alun Wyn Jones took over as the Lions triumphed for a 2-1 series win.
Lock Jones replaced Warburton as Wales captain for the 2017 Six Nations, when Warburton enjoyed an impressive tournament in the back row.
(Editing by Amlan Chakraborty)
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