The report, seen by the BBC, indicates a "worst-case scenario" of the new Sprucefield store would see spending in central Belfast reduce initially by 9%.
This represents annual spending of Â£47m. The report also ranked Belfast 16th as a "UK retail venue", below cities like Glasgow and Leeds in terms of luxury or high-end retail.
"Belfast should look to attract more upmarket retailers and the addition of John Lewis would be a major catalyst in delivering this," the report by London consultants Javelin added.
The report states John Lewis would "be very positive for Belfast", potentially increasing retail spending in the city by 17%.
John Lewis has almost 50 stores in the UK, but none in Northern Ireland.
It has always been thought to favour opening at Sprucefield, near Lisburn, but the report will only add to Belfast's desire to challenge that.
John Lewis has been mooted as a potential tenant in the city's major Royal Exchange project, currently in planning.
The site encompasses a swathe of properties between Royal Avenue and the Cathedral Quarter.
It was recently bought by London-based developer Castlebrooke Investments.
Belfast needs to improve the quality of its shops, with a John Lewis store seen as a "major catalyst" to rejuvenate the city's retail sector, according to a report for the city council.