Image copyright Reuters Image caption How do London schools look when compared with other global cities?

London's schools are falling behind many global competitors, according to an analysis of international tests.

The capital's schools have been held up as a showcase of rising standards.

But the University College London (UCL) Institute of Education study, using OECD Pisa test results, suggests they are weaker than those in many Asian cities and the rest of the UK.

However, the OECD's education director, Andreas Schleicher, has rejected the findings as "not credible".

Mr Schleicher says the London schools taking the Programme for International Student Assessment (Pisa) tests were not intended to be representative of the greater London area and such comparisons were invalid.

The Department for Education also cautioned about sample size and that the results were from several years ago.

But academics at the UCL Institute of Education say they have produced a robust sample of 42 schools by combining the results of two year groups taking the tests in London in 2009 and 2012.

Image copyright Reuters Image caption Shanghai remains the most successful school system in international tests

The analysis compares these London results, in reading, maths and science, with the performance of 15-year-olds in cities, regions and countries around the world.

It suggests if London entered the Pisa tests as a separate education system, pupils taking maths would be three years behind their counterparts in Shanghai, which has the world's top-ranking results.

And as well as being behind high-flyers in maths such as Singapore, Hong Kong, Finland and Poland, teenagers in London were below the level of counterparts in countries such as Spain, Italy, France, Slovenia and Latvia.

For reading, the analysis put London behind Madrid in Spain, Riga in Latvia and Massachusetts, the highest-achieving state in the United States.

Image caption Does the study cast doubts on London's role as a model for school improvement?

In science tests, London would be above the US state of Florida and Buenos Aires in Argentina, but behind Moscow in Russia and Quebec in Canada.

And in all three subjects, London was far behind the school systems of Shanghai, Hong Kong and Singapore.

The report also suggests London schools were below the UK national average, with girls and disadvantaged and ethnic minority pupils in London performing less well at Pisa tests compared with their GCSE results.

However, these Pisa tests were taken against a backdrop of London schools improving, and the Organisation for Economic Co-operation and Development (OECD) says the London schools taking them were, in any case, not a reliable sample for such regional comparisons.

Report author John Jerrim said the study was a "single piece in a much bigger jigsaw" and the success at GCSE in London schools was still a "cause for celebration".

But she added: "The global perspective is what we should focus on.

"We shouldn't just look to London as a benchmark, we should look at the wider world."

Munira Mirza, London's Deputy Mayor for Education and Culture, said it showed there should be no "complacency" in supporting London schools to improve.

"Whilst young Londoners get the best results in the country at GCSE, this research highlights the challenges still faced by London's disadvantaged students in achieving as well as their peers."

A Department for Education spokeswoman said: "London has been a real success story, particularly in improving outcomes for disadvantaged pupils, as the report authors acknowledge, and our reformed world-class qualifications are ensuring all pupils have the chance to fulfil their potential.

"This report is based on a small sample of schools, and uses data from 2012 and doesn't take into account the impact of many of our reforms, so should be treated with caution."