The letter comes from the Progress think-tank and its signatories are from nine former cabinet members, including Patricia Hewitt, Estelle Morris, Hilary Armstrong, Beverley Hughes and Caroline Flint.
It says: "We believe the Government must look again at the decision, carefully considering the full impact of removing the tax relief on parents, employers and the childcare sector."
It adds: "Surely this is not the time for us to remove a key support from hard-working families at the very point we need them at their most engaged and productive to fuel the recovery from recession. Crucially, in the run-up to an election, it will remove support for working parents and for businesses in key marginal constituencies."
Other signatories include the former Scotland Office minister David Cairns, former Europe minister Denis MacShane, and the former international development minister Sally Keeble.
In addition to this letter, 43 Labour MPs have backed a Commons motion warning the Government that it will worsen the gender pay gap, reduce opportunities and incentives for two parents to work, impair the quality and affordability of childcare.
Since announcing the plans at the Labour Party conference, Brown has attracted derision from all political parties.
The number of people that have signed a Downing Street petition calling on Brown to rethink scrapping childcare vouchers has broken 75,000 - making it the most popular e-petition on the number10.gov.uk website.
The Institute of Payroll Professionals yesterday said 80% of its members believe they do not think organisations will continue running childcare voucher schemes after tax exemptions are abolished.
