Treasury Minister David Gauke wrote to MPs insisting the measure will be legislated for before the 2015 general election.
It was seen as an effort to calm backbench tensions over the failure so far to implement the Conservative manifesto promise. But critics said the pledge of action "in due course" was wearing thin and called for a date to be set.
Under the Tory plan, non-working spouses and civil partners would be able to transfer some tax-free allowance to partners, saving up to £150.
The coalition agreement with the Liberal Democrats makes increasing the personal allowance to help low earners the personal tax priority. It also opens the way for the marriage break, by specifying that Lib Dems would be allowed to abstain in any Commons vote.
Tory ex-minister Tim Loughton is leading the push for the measure to be introduced quickly - and has tabled an amendment to the Finance Bill with the potential to cause a damaging revolt.
"An announcement on details of how we will legislate for this in this parliament will be made by the Chancellor in due course," he told them.
Mr Loughton told the Daily Telegraph that patience was wearing thin. He said: "There is only a certain amount of promises about 'in due course' that hard-working families can take."
Another former minister, Gerald Howarth, said he would be looking for it to be included in the Chancellor's Autumn Statement. He said: "We stuck to the commitment on overseas aid. The Government has forced through gay marriage. Why is this commitment such a difficult one to fulfil?"