Sky Sports reports that the Magpies are now favourites to sign the 25-year-old defender.
Newcastle are keen to bring in Mariappa as a replacement for Steven Taylor, who will be out of action with an injury for the reest of the season.
Mariappa, who has also been linked with Swansea City, has 18 months left on his current contract with Watford.
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Chelsea have struggled to build momentum in the opening stages of the campaign and are currently looking over their shoulders at the relegation zone.
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