Quiet right. Tax credits were introduced in 2003, and the immigration rules were subsequently modified in 2005. The rule change in 2005 affects couples where one partner is not subject to immigration control (i.e., a British citizen or permanent resident); and the areas the rule addresses are [1] working tax credits and [2] child tax credits. Under the rule, a couple may claim these tax credits without jeopardizing the immigration status of the person subject to immigration control.
Simply put, a married (unmarried, civil partnership) couple can claim child tax credits and/or working tax credits as long as one partner is a British citizen or otherwise exempt from immigration control.