John Lewis is an employee owned department store

Research carried out on behalf of the department store by Imperial College also shows regular customers are going there less because of the road charge.

But a survey by the store of its own shoppers showed about 51% were in favour of the �5 toll.

Transport for London's annual report is expected to say there has been no effect on trade in the West End.

Speaking on Wednesday, Sir Stuart Hampson, the John Lewis Partnership Chairman, said: "The question as to whether the congestion charge has had an impact on retailers is now beyond dispute and cannot be dismissed.

JOHN LEWIS FINDINGS Exit survey revealed 8.1% of customers were shopping less frequently at John Lewis Oxford Street since the introduction of congestion charging Postal and shop surveys show the charge is listed as the prime reason for shopping less frequently in Oxford Street When asked to select three possible changes to the congestion charging scheme, "making payment easier" was listed by 48.5%. The next most popular change was the "introduction of a charge free window", which was listed by 36.7% The postal showed 51.5% of respondents support congestion charging with 26.6% opposed to it. The Oxford Street exit survey showed 56.3% in favour

"A 5% drop in sales is significant for any retailer, particularly smaller businesses, and the mayor needs to take the concerns of retailers seriously when considering changes to the scheme."

Sir Stuart also called on the London mayor Ken Livingstone to abandon his plans for the western extension of the charging zone.

He added: "These figures show that we need to see a full economic impact assessment on extension before we are in a position to judge whether it is sensible.

"Not to do so risks inflicting serious economic damage on an even wider part of London, and there are good reasons to expect that the impact could be significantly greater than has been seen in the existing zone."

But Transport for London (TfL) says the charge has deterred 65,000 to 70,000 car trips, reduced traffic delays, improved reliability, lowered fuel use, and lessened pollution - leading to a benefit of �50m.

And TfL's annual report, which uses figures from independent analysts, says traders did suffer in the first half of 2003 but said the slow down was not the charge's fault and that it had begun before its introduction.

It quotes figures from the SPSL retail traffic index which shows the West End outperformed the rest of the UK in the second half of 2003 and has now reached levels previously seen in 2002.

John Lewis' research, which took place from January 2003 and January 2004, separates the impact of congestion charge from other factors affecting London in early 2003, such as the Central Line closure and the Iraq War.

TfL's second annual report is expected next week.