Melampsora euphorbiae

The primary stage of the rust Melampsora euphorbiae causes galls on various species of Euphorbia, commonly recorded as bright orange aecia or uredinia on Petty Spurge and Sun Spurge, and occasionally on other species of Spurge. There is no host alternation.  Stems and leaves become distorted and thickened. 

This is the only species that is said to cause galls on most species of Euphorbia, including non-native and naturalised species.  On the Cypress Spurge, Euphorbia cyparissias, a Uromyces species may cause galls.  

Melampsora euphorbiae-amygdaloidis may be present on Wood-spurge and Melampsora euphorbiae-dulcis on E dulcis and a few otehr non-native species that may be naturalised; these are probably (?) not gall-causers.  See 'Plant Parasites of Europe' website for more details.

host/gall

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.

Spurges (Euphorbia)

On 'Plant Parasites of Europe' website Melampsora helioscopiae is listed as a separate species affecting Sun Spurge; it is synonymous with M euphorbiae on the NBN.