The future of Waitrose's Marlow store was thrown into question this week after Sainsbury's secured a 28.5-year lease on the site.
The shock move means Waitrose faces being forced to vacate the premises when the lease expires in two years' time.
"We had been negotiating the leasehold renewal at Marlow when Sainsbury's acquired a superior leasehold interest in a site including the shop," said Nigel Keen, director of development. "This move by another retailer was a complete surprise."
Customers at the Marlow store were reportedly faced with long queues this week as manager Alex Hume broke the news to staff.
If the store is taken over, loyal Waitrose shoppers will have to travel four miles to Waitrose's Maidenhead store. Sainsbury's said that it had "no firm plans for the building".
Waitrose lost a battle last June to extend the store from 12,700 sq ft to 24,542 sq ft following opposition from local residents.