The Evangelical Alliance has criticised the Green Party for its investigation into one of its councillors after she spoke against gay marriage.

Brighton Councillor Christina Summers voted on grounds of Christian conscience against a motion approved by Brighton & Hove City Council in July to promote gay marriage. The Green Party is now considering her role and whether to expel her.

As the Green Party gather for their annual party conference, the Evangelical Alliance has challenged them on whether they are inclusive or extremist.

Dr Dave Landrum, director of advocacy at the Evangelical Alliance, said the Green Party's actions against Summers showed a "serious lack of thinking about what equality and diversity actually are".

"It also casts doubt on whether the party will be able to achieve their long-term goal of becoming a credible, mainstream influence in UK politics," he said.

Gay marriage is not legal in Britain but the Government announced plans earlier in the year to change the law to allow same-sex couples to marry, despite strong public opposition.

Dr Landrum called upon the party to prove that it is inclusive and capable of governing for all the people of the UK.

He said that a failure to exonerate Councillor Summers would effectively disbar Christians, Muslims and other religious people from Green politics and consign the party to a future as a "representative vehicle for an extremist minority".

"All the main political parties in the UK have Christian members, MPs, Peers and councillors who are opposed to the plans to redefine marriage," said Dr Landrum.

"This valuing of diversity and the common commitment to democratic politics is a hallmark of our inclusive, plural society.

"If the Green Party wants to join this club and be taken seriously in British politics, it needs to present a far more mature view of equality and diversity.

"Otherwise, if it fails to support Councillor Summers, in order for voters to understand what they would be voting for, it will need to provide a very detailed, intellectually rigorous account of what it thinks equality actually is, and how freedom of conscience relates to it.

"The party is at a crossroads, and the Alliance is calling on the new leader, Natalie Bennett, to address this issue as a matter of urgency."

CORRECTION: The original article published 08/09/12 incorrectly stated that Christina Summers had been suspended. We apologise for any confusion this may have caused.