A Government member is quitting in protest at plans to abolish the 10p tax rate, the BBC reported tonight.
Angela Smith, MP for Sheffield Hillsborough, is Parliamentary Private Secretary to Treasury Minister Yvette Cooper.
Ms Smith is said to have told colleagues she is resigning as Ms Cooper's aide because of her concerns over the tax move.
Prime Minister Gordon Brown is under mounting pressure from Labour MPs over the issue.
They are concerned about the impact on poorer people.
Former welfare reform minister Frank Field has warned that the PM faces a "mutiny" among Labour MPs over the issue when the Commons returns from its Easter recess next week.
Ms Smith was elected to Parliament at the 2005 General Election.