The former England man has been training with the Gunners since he walked out on Notts County in September, just weeks after signing a five-year deal with the League Two club.
A hidden article on Arsenal.com revealed he was signing and in line to play at Upton Park for the reserves, though club insiders said it was published in error.
It read: "Sol Campbell is back at Arsenal. The 35-year-old has re-signed for the Club he graced for five years between 2001 and 2006 and will make his comeback for the Reserves at West Ham on Tuesday night.
"Campbell was a key figure in the Arsene Wenger teams which brought so much success to Highbury in the first half of the last decade."