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Thelondonpaper has criticised the NRS over the measurement problems, saying it needs to work on the credibility of its data.

Ian Clark, thelondonpaper's general manager, said: "It is shocking that in January NRS managed to interview only 15 readers of thelondonpaper in a month when over 8m copies had been picked up by our readers."

Elaborating on the problems, NRS said that the December-May sample was insufficient because it cannot include responses gathered via the self-completion questionnaire method.

Roger Pratt, managing director of the NRS, said: "Self-completion questionnaire respondents are only included in the full quarterly database, and the April and May data about to be released therefore currently exclude them."

The NRS particularly relies on the self-completion questionnaire in London, where it is harder for it to find people to participate in its traditional surveyor home visit programme.

The traditional method has effectively failed to include enough freesheet readers to make the data significant.

Clark said: "Although the decline in the London response rate has recently been reversed by NRS, it remains unacceptably low and is 10% below the average for the rest of the country.

"NRS needs to increase its use of self-completion questionnaires and urgently consider other measures to better reach our audience of ABC1 under-35s -- a notoriously difficult audience to research."

The problems delay the first release of readership data for the London freesheets until September, meaning buyers only have ABC circulation data to go on.

The latest ABC data shows that in June thelondonpaper handed out 496,504 copies and the London Lite 388,442 copies.