A completion method for completing a uniform inner bore downhole completion system in a reservoir in a formation, comprising the steps of: - drilling a hole in the formation, - inserting a surface casing having an inner surface casing diameter (D i1), - cementing an outer face of the surface casing to the hole, - inserting a drilling head in the surface casing, - drilling a main bore extending from the surface casing, the main bore having a first part and a second part, the second part comprising an end of the main bore, and the main bore having an inner main bore diameter (D i2) which is substantially the same as the inner surface casing diameter of the surface casing, - retracting the drilling head from the main bore and the surface casing, - inserting a lower production casing in the second part of the main bore, - inserting the drilling head into the first part of the main bore below the surface casing, - drilling a lateral bore from the first part of the main bore, - retracting the drilling head from the lateral bore, - inserting a lateral production casing fully in the lateral bore, characterised in that the lower production casing having annular barriers and an inner diameter (D i3), and in that the lateral bore having an inner lateral bore diameter (D i4) which is substantially equal to the inner main bore diameter, and in that the lateral production casing having annular barriers and an inner diameter (D i5) which is substantially equal to the inner diameter of the lower production casing providing a uniform inner bore completion having the same inner diameter in all production casings, and in that the completion method further comprises the steps of: - setting at least one of the annular barriers of the lower production casing, - setting at least one of the annular barriers of the lateral production casing, - inserting a main production casing in the first part of the main bore, so that the main production casing is fluidly connected with the lower production casing, the main production casing having annular barriers and an inner diameter (D i6) which is substantially equal to the inner diameter of the lower production casing, - setting the annular barriers of the main production casing, and - providing an opening in the main production casing opposite the lateral production casing fluidly connecting the lateral production casing with the main production casing, so that the lower production casing, the main production casing and the lateral production casing are in fluid communication. - drilling a hole in the formation, - inserting a surface casing having an inner surface casing diameter (D i1), - cementing an outer face of the surface casing to the hole, - inserting a drilling head in the surface casing, - drilling a main bore extending from the surface casing, the main bore having a first part and a second part, the second part comprising an end of the main bore, and the main bore having an inner main bore diameter (D i2) which is substantially the same as the inner surface casing diameter of the surface casing, - retracting the drilling head from the main bore and the surface casing, - inserting a lower production casing in the second part of the main bore, - inserting the drilling head into the first part of the main bore below the surface casing, - drilling a lateral bore from the first part of the main bore, - retracting the drilling head from the lateral bore, - inserting a lateral production casing fully in the lateral bore, - setting at least one of the annular barriers of the lower production casing, - setting at least one of the annular barriers of the lateral production casing, - inserting a main production casing in the first part of the main bore, so that the main production casing is fluidly connected with the lower production casing, the main production casing having annular barriers and an inner diameter (D i6) which is substantially equal to the inner diameter of the lower production casing, - setting the annular barriers of the main production casing, and - providing an opening in the main production casing opposite the lateral production casing fluidly connecting the lateral production casing with the main production casing, so that the lower production casing, the main production casing and the lateral production casing are in fluid communication.