Treasury officials were on Friday accused of making "dubious" claims about national insurance rises.
The department provoked fury after saying on social media site X "we're not increasing the basic, higher, or additional rates of Income Tax, National Insurance or VAT".
Shadow Chancellor Mel Stride has written to Treasury Permanent Secretary James Bowler about the social media post from the department's official account, claiming they may have broken civil service guidance.
Mr Stride asked Mr Bowler whether they had been pressured to say this, in line with Labour's election manifesto.
Chancellor Rachel Reeves increased employer national insurance contributions to raise an extra £25bn - the main tax rise in the Budget.
And Paul Johnson, Director of the Institute for Fiscal Studies, said of the post on X: "Seriously? I know this was in the Red Book. And I'm no expert on rules covering what government depts can/cannot say. But at best this is dubious. And that's being generous. Increasing NI was the central tax raising measure in the Budget. What is the point in pushing this line?
The tax burden as a share of GDP is on track to reach the highest sustained level since records began over 300 years ago, it warns.
Shadow Chancellor Mel Stride said: "I have written to the Permanent Secretary of the Treasury over the Government's consistent use of misleading claims relating to their National Insurance hike.
"Labour promised not to raise NICs and they did. They should be honest about that."
In his letter, Mr Stride said: "Do you agree, by claiming the Government is not increasing National Insurance, this information is misleading?
"Can you confirm officials were not pressurised into publishing misleading information by the Treasury?
"Do you agree it is important that official Government social media posts adhere to the Civil Service guidance on social media which states 'accuracy' as a 'point to bear in mind' when posting?"
Analysts have warned that the employer NICs rise will be a de facto tax on workers as businesses will cut back on pay rises to cover their higher tax bills.
The Office for Budget Responsibility (OBR) warned in its Budget analysis that workers would eventually pay for four-fifths of the tax rise through lower pay.
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