About 300 million rural people, or nearly a quarter of the mainland's population, will move to urban areas over the next 20 years and it is critical that the central government frames national development policies with this mass migration in mind, a UN-sponsored report has found. Such policies were crucial to sustaining the rapid pace of urbanisation because many cities would be facing limited natural resources such as land and water supplies, Xinhua quoted the report as saying. The joint study, funded by the UN Development Programme and conducted by the Chinese Academy of Social Sciences, found that mainland cities lagged behind those in developed countries in their contribution to the national economy. It also found low productivity rates and high densities in mainland cities made them unable to support future growth. The preliminary release of the report follows recent calls by prominent Chinese Academy of Sciences geographical economist Lu Dadao to slow the pace of urbanisation. Mr Lu said existing policy focused on massive construction projects, which were often under-utilised and deprived farmers of land. The UN report predicted that in four years about 125 mainland cities would have a population of more than 1 million and, of these, about 50 would have more than 2 million people. But limited natural resources would create a bottleneck in this growth, the report said. The government should strengthen city planning and find innovative ways to save resources, such as power and fuel by providing better public transport and encouraging the use of smaller vehicles. The report is based on the research of dozens of academic experts who, over five years, surveyed five cities on the mainland and looked at factors such as natural resources, living environment, public safety and public finances. The aim of the work was to find a strategy for urban planning, development and management for the mainland.