The correspondence comes as a consulting company, commissioned by Ulster GAA, estimates that the value of five potential Euro 2028 games played at the venue would be worth £106m to the Northern Ireland economy.
Grant Thornton estimate that some £52m would come from accommodation and a further £54m from bars restaurants and other retail outlets that would benefit from the games being staged in Belfast.
The funding of the project, which has been ongoing for more than a decade now, has reached a deadlock as the deadline to commence works looms.
The British Government has not committed to bridging the funding gap with the full cost subject to much speculation, up to £308m in one case.
Casement Park is one of 10 venues selected for Euro 2028 at which 51 games will be played. The economic impact is estimated to be £2.5 billion, which would work out as a £255m pro rata contribution for Northern Ireland. But the report commissioned by Ulster GAA is more measured.
It focuses on economic and tourism benefits only and does not not reflect the contribution of the construction of the stadium or hosting major GAA games and other events that will take place in the years ahead.
So far the Assembly remains committed to £62.5m in funding, the Irish Government pledged around £43m in February, while the GAA is putting up £15m.
Ulster GAA secretary Brian McAvoy said the letter to Prime Minister Sunak expressed “urgency.”
“We want and need to see progress, now, on the long-standing commitments given to deliver Casement Park,” he said in a statement.
“In the joint letter we thanked Prime Minister Sunak and the Government for their previous public commitments to invest in the project.
“We also called for clarity on the Casement Park and Euro 2028 funding position to enable the NI Executive to, then, take the necessary steps to deliver this landmark stadium. With the announcement of the impending dissolution of the UK Parliament within days this matter is now beyond urgent.
“If Euro 2028 can be hosted in Belfast, what a powerful message it would send out on the 30th anniversary of the Good Friday Agreement in 2028, shining a global light on the progress our community has made in building peace, prosperity and reconciliation.”
