Gatland is due to announce the full squad at midday with a number of high-profile players, including England captain Dylan Hartley, due to miss out.
And there had been talk ahead of the 2017 Six Nations that Warburton, who led the Lions in Australia four years ago, might not be given the role again after stepping down from skippering Wales to concentrate on his own form.
Sam Warburton led the British & Irish Lions in New Zealand
Sam Warburton appears to have been taking publicity shots with the coaching staff this morning
Warburton subsequently excelled even as Wales struggled to stamp their authority on the tournament although even then he was not expected to be handed the captaincy after Gatland publicly stated he would prefer a candidate who led their country.
However, Express Sport understands the Kiwi coach was ready to make an exception for such a trusted individual as Warburton.
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And now pictures of him posing for publicity shots this morning appear to have confirmed it.
Unlike other squad announcements, players are not told in advance of their inclusion in the Lions squad except for the captain.
The CEO of World Rugby also added his weight to the rumours of Warburton's appointment.
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"Many congratulations to Sam Warburton on becoming only the second player, after Martin Johnson, to captain two Lions tours," Brett Gosper said on Twitter.