The partnership, which runs the eponymous department stores business and up-market supermarket Waitrose, reported a loss before its partner bonus, tax and one-off costs of 25.9 million pounds.
John Lewis has said the main risk to the partnership from an unmanaged Brexit was a sharp fall in consumer confidence and the impact that would have on trading.
"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," said Charlie Mayfield, chairman of the John Lewis Partnership.
"Brexit continues to weigh on consumer sentiment at a crucial time for the sector as we enter the peak trading period."
Prime Minister Boris Johnson has pledged to take Britain out of the European Union with or without a deal on Oct. 31, and has said he will not ask for a delay despite lawmakers voting that he seeks one to avoid a no-deal Brexit.
At the same time, Britain's department store sector has been under intense pressure from weak demand and rising costs for several years.
British Home Stores (BHS) went bust in 2016, House of Fraser was bought out of administration last year by Mike Ashley's Sports Direct, and Debenhams went into administration in April and is now owned by its lenders.
For John Lewis, operating losses before exceptionals for its department store chain increased by 42.5 million pounds to 61.8 million pounds in the six months to July 27.
That contrasted with a rise in Waitrose's operating profit before exceptionals of 14.1 million pounds to 110.1 million pounds.
(Reporting by Alistair Smout, additional reporting by James Davey, Editing by Paul Sandle and Tom Hogue)
