Context : The study of the radial variations of metallicity across the Galactic disc is a powerful method for understanding the history of star formation and chemical evolution of the Milky Way .
Although several studies about gradients have been performed so far , the knowledge of the Galactic antincentre is still poor .

Aims : This work aims to determine accurately the physical and chemical properties of a sample of H ii regions located at R _ { G } > 11 kpc and to study the radial distribution of abundances in the outermost part of the Galaxy disc .

Methods : We carried out new optical spectroscopic observations of nine H ii regions with the William Herschel Telescope covering the spectral range from 3500 Å to 10100Å .
In addition , we increased the sample by searching the literature for optical observations of regions towards the Galactic anticentre , re-analysing them to obtain a single sample of 23 objects to be processed in a homogeneous and consistent manner .
The total sample distribution covers the Galactocentric radius from 11 kpc to 18 kpc .

Results : Emission line ratios were used to determine accurate electron densities and temperatures of several ionic species in 13 H ii regions .
These physical parameters were applied to the spectra to determine direct total chemical abundances .
For those regions without direct estimations of temperature , chemical abundances were derived by performing tailor-made photoionisation models and/or by using an empirical relation obtained from radio recombination and optical temperatures .

We performed weighted least-squares fits to the distribution of the derived abundances along the Galactocentric distances to study the radial gradients of metallicity across the outermost part of the MW .
The distributions O/H , N/H , S/H , and Ar/H towards the anticentre can be represented by decreasing linear radial gradients , while in the case of N/O abundances the radial distribution is better fitted with a two-zone model .
The He/H radial gradient is presented here for the first time ; we find a slope that is not significantly different from zero .
The derived gradient for oxygen shows a clear decrease with distance with a slope of -0.053 \pm 0.009 dex kpc ^ { -1 } .
Although a shallower slope at large Galactocentric distances is suggested by our data , the flattening of the distribution can not be confirmed and more objects towards the anticentre need to be studied in order to establish the true form of the metallicity gradient .

Conclusions :