Capsid protein self-assembles, with the help of the scaffolding protein gp11, to form an icosahedral capsid with a T=7 symmetry, about 61 nm in diameter. The capsid encapsulates the genomic DNA. Interacts with the scaffolding protein gp11 (PubMed:10656821). Interacts with the decoration protein gp12 (PubMed:22514336). Part of the capsid. Belongs to the lambda phage major capsid protein family.