The concept of an integrated spectrometer using coaxially joined fibre-segments is presented. The obtained spectrum is intrinsically cross-dispersed with respect to the free spectral ranges in both lateral directions. The key element is a multi-core fibre in which the N × N cores are arranged in a compact helical bundle spiralling around the fibre central axis. Dispersive properties of the core bundle and the mechanism for diffractive image formation are investigated. The spectral resolution and optical loss in relation to the fibre footprint is analysed. To resolve 25 channels with 1.6 nm spacing in the C-band, the device can be contained in a volume of 1-mm-diameter cladding over a length of ∼10 cm. Furthermore, it is shown that an inverse taper structure can reduce the device loss dramatically by improving the wave-front overlap without exciting high-order modes, a feature that is not achievable in conventional planar spectrometers.