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Gillingham will receive additional tickets from Portsmouth after selling out their initial allocation.
All 1,322 tickets for Saturday's game at Fratton Park have been snapped up, prompting the Gills to request more.
The club have subsequently been given an additional 500 tickets - up from their first request of 200 more - and they are on sale now.
Both teams have a chance of making the play-offs, albeit a long-shot. The Gills are currently three points adrift of Portsmouth, who are another four adrift of those play-off places.
Saturday's game is 'all-ticket' and there will be no sales on the day.
Ticket prices for fans are set at £23 for adults, £17 for over-63s, £15 for 18-22-year-olds and £10 for unaccompanied under-18s. Under-18s accompanied by a paying adult/senior pay £5.
Fans can buy their ticket by calling the club on 01634 300000 tomorrow or visiting the ticket office before 4pm on Friday, when they go off sale.