The man behind the influential international league table that placed Shanghai above all other school systems has revealed that his study does not include results of more than a quarter of pupils in the Chinese city.

The admission from Andreas Schleicher that the Programme for International Student Assessment (Pisa) only covers 73 per cent of Shanghai’s 15-year-olds will provide succour to sceptics who think that the city’s results are too good to be true.

The figure, which he gave while speaking to MPs in London this morning, is markedly lower than the 79 per cent he quoted in an article in December.

But Mr Schleicher, who is the deputy director for education and skills at the Organisation for Economic Co-operation and Development (OECD), today insisted that the more important finding was that the children of menial workers who did take the test outperformed children from professional families in the West.

Doubts have been expressed about the apparently outstanding performance of China’s economic powerhouse city since December, when the latest Pisa survey showed the difference between Shanghai’s maths score and the average for industrialised countries was “the equivalent of nearly three years of schooling”.

Tom Loveless, a former teacher and Harvard University policy professor, quickly cast doubt over whether the city’s poorest young people were properly represented in the Pisa results.

Writing for the Brookings Institution, a respected American think tank, he noted that the children of Shanghai’s millions of rural migrants will not have the official residency status needed, until very recently, to attend the city’s high schools under China’s controversial hukou registration system.

The academic went on to note that Pisa’s figure for the number of 15-year-olds in Shanghai was suspiciously small.

Mr Schleicher and his colleagues have previously mounted a vigorous defence against such claims.

But today, when a member of the Commons Education Select Committee, suggested that the same figure was “very, very low given the overall population of Shanghai”, Mr Schleicher admitted: “You are right.”

“We have a population coverage in Shanghai of 73 per cent, which is low,” he said. He later described it as “sort of low by OECD standards”.

Mr Schleicher then seemed to contradict himself, saying: “When you look at the total number of students in the OECD it is pretty much average. I don’t think Shanghai is an exception.”

But asked about a comparable figure for European Union countries, he said “in many countries, it is about 10 per cent [of 15-year-olds] that are not covered by Pisa”.

Mr Schleicher also claimed that Pisa’s coverage of Shanghai was “not dramatically lower” than for the United States. But an article he has co-written gave the American figure as 89 per cent - much higher than the 73 per cent he quoted for Shanghai today.

Pisa has portrayed Shanghai as a “high equity” schools system. That fact, combined with its high test scores, has already prompted a visit from a British ministerial delegation to the city last week.

But this morning, Mr Schleicher conceded the city still had a “significant fraction” of children who were not part of its schools system.