Even after the Greens' recent surge, which has seen them poll as high as 8 per cent (they won just 1 per cent in 2010), the party still trails far behind Ukip. But a fascinating new poll by YouGov for the Times's Red Box shows how Natalie Bennett's band could overtake Nigel Farage's.

Asked which party they would vote for if all candidates had a chance of winning in their constituency, 26 per cent said they would "likely" support the Greens, ahead of Ukip on 24 per cent. Most likely owing to Ukip's toxic status among many centrist and liberal voters, fewer are willing to consider supporting it than the left-wing alternative. The Tories and Labour were on 35 per cent each, with the Lib Dems in last place on 16 per cent.

The polls shows the potential for the Greens to split the British left in the manner of its European counterparts, a danger Labour is alive to. As I recently revealed, the party has established an electoral unit, led by Sadiq Khan, to counter defections to its rival. Labour fears that the party could cost it as many as 17 marginal seats, in liberal-leaning areas such as Brighton, Bristol, Cambridge and Norwich.

The greatest obstacle to the Greens' advancement remains, of course, Britain's antiquated first-past-the-post voting system. Only in a handful of constituencies can the party claim to be in contention for first place, allowing its competitors to warn of the danger of "wasted votes". But that 26 per cent are prepared to contemplate voting for the Greens shows the extent to which traditional loyalties are fraying.