Everton have given the Hammers permission to talk to the veteran defender about a move to Upton Park, says the 59-year-old, who is in the middle of an injury crisis at the back
The Netherlands international has not featured for the Toffees in the top flight this season and is reportedly keen to leave Goodison Park in order to boost his chances of featuring at World Cup 2014.
Allardyce confirmed that Roberto Martinez has agreed to let the 30-year-old move to Upton Park, with personal terms only standing between a deal being completed.
A spate of injuries has left West Ham heading into 2014 with no fit centre-backs, with George McCartney and Alou Diarra starting in the middle against Fulham on New Year's Day.
"We've had the OK from Everton and we're talking to his agent at the moment," Allardyce told BBC Sport on Wednesday, "so hopefully that will happen in the next day or so."