Trust urged to improve standards at existing 17 schools and two due to open in 2010The government has banned the largest sponsor of academies from taking on new schools until it dramatically improve the ones it already runs, the Guardian has learned.The United Learning Trust (ULT) was called into the office of the schools secretary, Ed Balls, last week and told it could not sponsor any more schools until it had driven up standards in the 17 it runs and the two due to launch next September.It follows the spectacular failure of ULT's Sheffield academies, which have been plagued with behavioural problems, have struggled to improve results and were judged inadequate by Ofsted inspectors.The problems raise questions about government – ...