According to data from the combined number of unique visitors to the Thetimes.co.uk and Thesundaytimes.co.uk fell from 2.79 million in May to 2.22 million in June and to 1.61 million in July.
News International made registration compulsory and began redirecting users from the old site on 15 June, and began charging for access to the two new sites on 2 July.
Paul Thomas, media consultant and former press director at Mindshare, said: "These figures will be a concern for advertisers. Brands will be questioning whether The Times is a digital arm worth investing in."
Few prominent brands have advertised on the sites since the paid strategy came into effect, with Lloyds TSB and Holiday Place among the major brands to advertise behind the paywall.
The data has prompted media buyers to urge News International to reveal registration numbers if it wants to lure more marketing activity to the sites.
Justin Barnes, media director at Carat, said: "From an advertiser point of view, the new numbers reveal The Times is a site that only brands running campaigns for a specific premium advertising audience will be interested in.
"News International needs to disclose internal data that reveals registration levels if it wants to lure in more advertising across The Times sites."
However, a source at News International told Media Week that there had been "a positive reaction from advertisers and advertisers have been re-booking". The source did not disclose further details.
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