Chancellor Philip Hammond has been urged to publish the costs of any deals made with the DUP to prop up a Tory government.
Talks are under way between the Tories and the Northern Ireland party over a potential alliance as Theresa May needs their 10 MPs to govern after her majority was wiped out in the general election.
However, Labour’s shadow chancellor John McDonnell has raised concerns over reports the DUP want to end airport tax on visitors to Northern Ireland, which generated around £90 million in 2015/16, according to HMRC estimates.
Abolishing Air Passenger Duty (APD) is one of the DUP’s key demands as it pits Northern Ireland unfavourably against the Republic of Ireland, where the duty has been abolished.
In a letter to the chancellor, Mr McDonnell said: “I am writing to ask whether the government has been asked for this measure by the DUP, has agreed to it or is considering it.
“In the interest of basic openness and transparency in government I am writing to ask that you publish any financial measures that have been discussed with the DUP and set out the financial implications of any measures agreed with this party.
“I look forward to a prompt response given the considerable public interest in this matter and its consequences for our public finances.”
The prime minister has said she is confident of getting the Queen’s Speech through the Commons, regardless of whether a deal is reached with the DUP by the time of the State Opening of Parliament on Wednesday.
A Number 10 spokesman would not comment on the ongoing talks, which are understood to focus on support for key Commons votes rather than a full coalition between the parties.