Chelsea season-ticket holders might be unable to attend Saturday's Premier League opener against Crystal Palace because of over-running rail-seating work.
The club will confirm on Friday afternoon whether Stamford Bridge will be able to operate at full capacity for the first game of the 2021/22 season.
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Supporters with seats in the Matthew Harding Lower Stand are most likely to be affected.
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A Chelsea statement read: "Over the summer the club has been replacing over 12,000 seats in the Shed Upper and Lower Stand and the Matthew Harding Lower Stand with rail seating.
"The Shed Upper and Lower are complete and the major parts of the Matthew Harding Lower are also on track. However, the manufacturer is currently unable to guarantee the supply and fitting of some remaining specialist parts in sections in the Matthew Harding Lower before the match against Crystal Palace on Saturday.
"This means certain sections of the stand may need to remain closed. We are doing everything we can to prevent this, including working 24 hours at the factory and on site, but we wanted to notify supporters of this possibility, so they can plan accordingly.
"The areas most at risk are: the Matthew Harding Lower blocks 8, 9, 14, 15 and 16. We continue to work with the manufacturer and our teams on site through tonight and tomorrow, to ensure as many seats as possible are available.
"In the event that we cannot complete these seats those affected season-ticket holders will receive a pro-rata refund for the match plus a refund of any reasonable pre-booked travel costs. Affected members will receive a full refund, plus a refund of any reasonable pre-booked travel costs.
"In addition, after discussions with the Fans Forum and several supporter groups, affected season-ticket holders and members will also receive a complimentary ticket to one of the first three home Champions League group stage games this season. Those affected supporters will be contacted next week regarding the process for redeeming this.
"We sincerely apologise for this situation, we should have done better. We have tried everything possible to avoid it, but due to the bespoke nature of the fittings we have no alternative but to wait and see if the supplier can meet our manufacturing and fitting deadlines.
"A definitive announcement on which seats are affected will follow at 3pm Friday. This announcement will be published on the club website, app and social media channels. We will also contact supporters affected by email and supporters are advised to look out for this communication. Please do not contact the ticket office as they have no further information at this point in time."