Politicians from the main NI parties have gone to London to fight plans to downgrade Tyrone County Hospital.
Campaigners warn lives could be put at risk
Health minister Shaun Woodward proposed earlier this year to remove acute surgery and accident and emergency services from the Omagh hospital.
Services would be provided in a new Enniskillen hospital. In October, he announced six weeks public consultation on the issue.
The politicians presented a petition at Downing Street.
Danny McSorley, chairman of the Hospital Campaign Steering Group, said a downgraded facility would put lives at risk.
"If this decision is not changed by the minister, people will die or will have to put up with unnecessary suffering," he said.
"We feel so strongly about this that our MLAs are going together to work as a group and speak with one voice to lobby the prime minister to have this decision reversed."
The County Tyrone delegation included Thomas Buchanan of the DUP, the UUP's Derek Hussey, Eugene McMenamin of the SDLP, Pat Doherty, who is the area's Sinn Fein MP, and his party colleague Barry McElduff.
Dr Kieran Deeny, who was elected to the assembly as an independent on the single issue of the hospital's future, also went to Downing Street.
Donna Marie McGillion, who was seriously injured in the 1998 Omagh bombing, travelled with the delegation to hand in her own letter to Tony Blair.