LONDON (Reuters) - Britain’s government is under growing pressure to abandon plans to reopen most primary schools in England, with one teachers’ union demanding evidence it is safe to open and another telling members they do not have to work in an unsafe environment.Earlier this week, the government announced that most primary schools will open as planned on Jan. 4, while the start of term for secondary school children will be Jan. 11 for pupils sitting exams, and Jan. 18 for the rest. The National Association of Headteachers, England’s largest headteachers’ union, said on Saturday they had instructed lawyers to write to the government demanding that it reveals the data behind its decision to reopen schools. The National Education Union called on the government to move learning online in all primary schools in England for the first few weeks of the month. It has also issued advice to its members reminding them of their legal right not to have to work in an unsafe environment. Prime Minister Boris Johnson’s government has been criticised for frequent reversals during the pandemic, including delaying lockdown during the first wave in March and abandoning a system to award school grades without exams.Earlier this week, in a hastily arranged statement to parliament on bringing children back to school this term, the education minister Gavin Williamson said that the majority of primary schools in England would open as planned on Monday. A few days later, he announced that all London primary schools will remain shut during the first weeks of January.