Context :

Aims : The aim of this paper is to study the evolution of the broadband spectrum of one of the brightest and nearest quasars 3C 273 .

Methods : We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM-Newton monitoring of the blazar 3C 273 in 2003–2005 in the UV , X-ray and soft \gamma -ray bands and study the results in the context of the long-term evolution of the source .

Results : The 0.2-100 keV spectrum of the source is well fitted by a combination of a soft cut-off power law and a hard power law .
No improvement of the fit is achieved if one replaces the soft cut-off power law by either a blackbody , or a disk reflection model .
During the observation period the source has reached the historically softest state in the hard X-ray domain with a photon index \Gamma = 1.82 \pm 0.01 .
Comparing our data with available archived X-ray data from previous years , we find a secular evolution of the source toward softer X-ray emission ( the photon index has increased by \Delta \Gamma \simeq 0.3 - 0.4 over the last thirty years ) .
We argue that existing theoretical models have to be significantly modified to account for the observed spectral evolution of the source .

Conclusions :