Labour was accused of being incapable of being straight with the British public after dodging questions over its tax plans.
Sir Keir Starmer promised he would not hike up taxes on “working people” during the general election and insisted VAT, national insurance and income tax would be left untouched.
But the government has refused formal requests to explain who is included in the group and has also hinted the NI for employers will go up.
Shadow Treasury minister John Glen asked the Cabinet Office for an explanation of how it defines a working person and whether pensioners are included.
But a response to the parliamentary question is now overdue and ministers refuse to give an explanation.
Health Minister Stephen Kinnock said “definitions have to be seen in the round” and it would not be “appropriate” for him to speculate.
A Conservative Party spokesman said: “This Labour government is incapable of being straight with the British public.
“Time and time again they promised not to raise taxes on working people, and now just nine days from their first budget they still can’t even say what that means.
“They must urgently clarify who will bear the brunt of their tax plans or once again face accusations that they are gearing up to break their promises to the British people.”
Chancellor Rachel Reeves insists the Cabinet is united behind her spending plans despite reports of division.
Several ministers wrote letters to the Prime Minister raising concerns about the scale of cuts being demanded in the scramble to find £40 billion.
Ms Reeves said the final settlements had been confirmed but said it had been “right for all Cabinet members to want to get the best settlement for their departments”.
She added that settling departmental budgets was “an important achievement and shows the determination of this Government to get a grip of the public finances, and shows that we are as one, united in fixing the mess that the Conservatives left for us”.
Ms Reeves insisted her Budget would be “honest” about the state of the public finances and what needed to be done to put them “on a firm footing” while doing “everything within our power” to protect living standards and the NHS.
The Chancellor is also expected to give extra cash to boost the number of council houses being built.