Image copyright Getty Images

Green Party MP Caroline Lucas is to call for a "progressive alliance" with the SNP as her party holds what it says is its largest ever meeting.

At a spring conference in Liverpool, Ms Lucas will say the two parties could "forge a new grouping in Parliament".

The Greens are polling in similar numbers to the Lib Dems and say their membership quadrupled in the past year.

But the party's leader Natalie Bennett has faced criticism of her performance in recent weeks.

She struggled in a radio interview to explain how a flagship policy to build 500,000 new social rented homes would be funded, later attributing her performance to an "excruciating mind blank".

'Excruciating'

The leader of the Green Party on Liverpool council, John Coyne, said he had been "taken aback" by her "excruciating" performance and she had clearly been "underprepared".

He told the BBC that it wouldn't happen again as Ms Bennett - who could take part in a TV debate with other party leaders should they go ahead - would in future rehearse her performance in mock interviews.

"It's a failure that she was underprepared we know why that happened and we know it will be fixed for the future," Mr Coyne - a former Liberal Democrat who is now chair of the Green Councillors Association - said.

"It was a serious failure and we thought it might be damaging to us but one thing that perhaps is saving us from that is people who are attracted to the Green Party tend to have a more generous disposition anyway tend to be able to forgive a mistake."

In her conference speech, Ms Bennett will say her party offers a "message of hope and real change" and would create "a political system that puts the public first".

'Distinctive values'

The Green Party in England and Wales is fielding its largest ever slate of candidates, aiming to stand in 90% of parliamentary seats on 7 May. The Green parties in Scotland and Northern Ireland are separate organisations.

In her address, former leader Ms Lucas will say the Green Party should join forces with the SNP in the next Parliament to maximise their influence on the next government.

"With the rise of the SNP, and with our own Green surge, we have the chance to forge a new grouping in Parliament. A progressive alliance.

"Of course, in Scotland and in Wales we'll be fighting hard for our distinctive values and policies. Just as we do against those individual Labour and even Lib Dem candidates with whom we have something in common.

"That's the nature of British politics under the first past the post system."

Policy guide: Key priorities What are the top issues for each political party at the 2015 general election?

The SNP is riding high in the polls, threatening even the safest Labour seat in Scotland, according to recent opinion polls published by Tory peer Lord Ashcroft .