The Green party's mayoral candidate is celebrating a historic breakthrough for the party in British politics after it clinched three seats in the Greater London Assembly.

While defeated in the mayoral race, Darren Johnson will enter the assembly through the London-wide top-up list of candidates. He praised Ken Livingstone for throwing his weight behind the party two weeks ago.

Mr Livingstone, the first directly elected mayor of London, called on his supporters to back the Greens in the latter stages of the campaign, saying that he wanted the party to have a strong presence in the assembly.

With both Labour and the Conservatives holding nine assembly seats and the Liberal Democrats four, the Greens could prevent the big two from attempting to use wrecking tactics to paralyse a Livingstone administration.

"It's a very exciting day for the Green party," Mr Johnson told BBC News Online.

"We'd been campaigning for two seats but if we do get more than that, it will be a great breakthrough."

Mr Johnson said the support of the electorate proved that the GLA's agenda was that of the Green party - delivering change on transport, the environment and planning across Europe's largest city.

"It would be ridiculous to think about trying to do that without a strong Green voice there."

On Mr Livingstone's recommendation to his own supporters to back Green top-up list, Mr Johnson said: "Ken's backing of our GLA top-up list gave a boost to our campaign, but it must also be recognised that there's strong support for the Greens."

Turning to what he described as "frenzied speculation" about him becoming Livingstone's deputy mayor, Mr Johnson reiterated that he had held no "informal or formal" discussions with the prospective mayor.

"But I've always said that if it gives us an opportunity to push the Green agenda then yes, of course I'd be interested in being the deputy."