The John Lewis Partnership, which owns the department store chain and Waitrose supermarkets, has fallen to a half-year loss and said a no-deal Brexit would have a “significant” impact.
The group reported a loss of £25.9m, blaming a drop in sales.
It said “subdued consumer confidence” in the first half of the year had taken its toll on results.
John Lewis pointed to “soft demand” for its home and electrical goods as a particular weak spot.
The partnership, which normally makes most of its profits in the second half of the year, said it was making preparations for a no-deal Brexit.
However, the partnership’s chairman, Charlie Mayfield, said: “Should the UK leave the EU without a deal, we expect the effect to be significant and it will not be possible to mitigate that impact.
“Brexit continues to weigh on consumer sentiment at a crucial time for the sector as we enter the peak trading period.”
Salary increases and an IT overhaul also ate into the group’s profits, John Lewis said.
Amid what it described as a “weak grocery market” sales at Waitrose slipped slightly to £3.4bn.
However, the supermarket chain also reported a 10.7% growth in online sales, which the partnership said was “well ahead of the market”.
At the John Lewis department store business, total sales were £2.1bn, down 1.8%.