The PM said his “thoughts and prayers go out to his family and friends”.
Mr Cameron also stressed the British Government “stood fully behind” the Chief Constable and his officers as they work to protect Northern Ireland from terrorism.
I utterly condemn the murder of a young police officer today in Omagh, who had dedicated himself to serving the entire community of Northern Ireland.
Our thoughts and prayers go out to his family and his friends. This is a terrible tragedy for all who knew him and served with him, and for a town that had already suffered so much.
Those who carried out this wicked and cowardly crime will never succeed in dragging Northern Ireland back to a dark and bloody past. Their actions are rejected by the overwhelming majority of people right from all parts of the community.
I know that the Chief Constable of the PSNI will not rest until the perpetrators have been brought to justice.
For our part the British Government stands fully behind the Chief Constable and his officers as they work to protect Northern Ireland from terrorism.
And we, with our partners in the Northern Ireland Executive and the Irish Government, stand four square behind the people of Northern Ireland who have said time and again they want a peaceful, shared future.