DUP leader Gavin Robinson said he has pressed the Chancellor for a better financial settlement for Northern Ireland.
He said he also asked Rachel Reeves for borrowing powers that would allow Northern Ireland to build more homes, improve infrastructure and support economic growth.
It comes as the Stormont Executive remains unable to agree a new three-year budget almost two months into the new financial year as ministers complain their departments need more funding.
Mr Robinson said his party colleague, Education Minister Paul Givan is facing a funding shortfall running into hundreds of millions of pounds, which could lead to teaching posts lost, larger class sizes and reduced support for children with additional needs.
In an email to DUP party members, Mr Robinson said he heard from business leaders this week that while Northern Ireland has huge potential, it is being held back by planning delays, underinvestment and systems that simply do not work quickly enough.
He described a “wider reality which cannot be ignored”, that “Northern Ireland is being asked to deliver first-class public services with second-class funding”.
Mr Robinson said people are feeling the strain, from patients waiting longer than they should, local services disappearing and deteriorating roads.
He said he has been making this case directly to Government.
“I met the Prime Minister and made it clear that Northern Ireland is part of the United Kingdom and must be funded fairly as part of it.
“A strong Union must mean a fair Union,” he said.
“This week we again pressed the Chancellor for a better financial settlement and for borrowing powers that would allow Northern Ireland to build more homes, improve infrastructure and support economic growth.”
“At the same time, Government must get its own house in order.
“Around £350 million is lost every year through fraud and error, while inefficiency and unacceptable absenteeism continue within parts of the civil service.”
He went on: “Working families and businesses across Northern Ireland are already under enough pressure.
“They do not need new charges or stealth taxes from Stormont on top of continued Treasury underfunding.
“Talent, hard work and potential, Northern Ireland has them in abundance.
“What has been missing for too long is government willing to match that ambition with proper investment.”
“The UK Government has provided a £19.3 billion per year on average funding settlement for the Spending Review period, the largest in real-terms in the history of devolution.
“It is the responsibility of the Executive to manage that carefully.
“In addition to this, the Chancellor has announced over £750 million additional funding in Barnett Consequentials.”
“The Secretary of State is engaging regularly with the Minister for Finance on the budget, and it is clear he faces significant challenges in getting agreement from the Executive, and has asked for additional funding from the Government.
“Therefore, the Executive needs to come forward with a detailed strategic plan for how they will manage their finances and move towards sustainability.”
Northern Ireland Education Minister Paul Givan (left) walks with DUP leader Gavin Robinson (Liam McBurney/PA)