Labour want figures for all deals struck between DUP and Tories
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 £90m 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 Mr Hammond, 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 formal deal has yet to be secured but Tory sources have said there is a "broad agreement" on the principles of the speech, and State Opening will now take place on Wednesday - two days later than originally scheduled.
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.
Meanwhile, it has emerged that next year's Queen's Speech is being ditched by the Government to ease the way in Parliament for new Brexit laws.
In a highly unusual move, the parliamentary session is being doubled to two years.
Commons leader Andrea Leadsom said it would give MPs and peers the maximum time possible to scrutinise legislation taking the UK out of the European Union. It means the Government will not put forward a new legislative programme next year.
It comes amid growing pressure on Mrs May over her future as Prime Minister.
Tory backbencher Heidi Allen told The Sunday Times the country wanted a "leader and a party that will carry us through this most turbulent of periods but care about the little man".
"We have to change, and if we don't we deserve to die," she added.
Former Brexit minister David Jones, who was sacked in Mrs May's reshuffle, warned the PM not to row back on the exit strategy taking Britain out of the EU, telling the newspaper it would be a "betrayal of trust".
Extending the new parliamentary session will allow MPs and peers to examine Brexit laws as well as domestic reforms in depth, according to the Government. The Government said the Great Repeal Bill, which converts EU laws into British legislation, will be considered alongside other Brexit laws.