The report will predict how temperature and rainfall are likely to change at regional and local scales.
Scientists believe winters will be wetter, particularly in the north, and summers drier, especially in the south.
The projected impacts are "worse than the government had feared," according to a source familiar with the project.
The government hopes the UK Climate Projections 2009 report (UKCP09) will allow citizens, local authorities and businesses to plan better for future decades.
"The picture it paints is an alarming one"