LONDON (Reuters) - Prime Minister Theresa May unleashed a scathing attack on Britain’s main opposition party on Thursday, launching a campaign for local elections next month that will be the biggest electoral test so far of her leadership and her plan to leave the EU.

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Pressing home her advantage over Labour in opinion polls, she told voters to choose between the “competence” of her Conservatives or the “chaos” of the rest in the May 4 elections when voters across Britain will elect thousands of local authority councillors and six new mayors.

The ballot will also show whether opposition Labour has managed to defy the opinion polls, where the party’s standing has sunk since veteran lawmaker Jeremy Corbyn took over 18 months ago to lead the party further to the left.

With little formal opposition in parliament, May, who has yet to face election after being appointed prime minister following Britain’s referendum on EU membership, has been able to claim much of the political center ground - something she must try to maintain before parliamentary elections in 2020.

“When you look at it closely, these local elections present a clear and informative choice: the competence of a strong Conservative council, focused on the priorities of local people, keeping local taxes down and delivering high quality local services,” she told supporters in the central city of Nottingham.

“Or the chaos and disarray of the rest – political parties motivated not by what is best for local areas, but what best for their own partisan political interest, and without a plan for our country or our local communities – just a recipe for chaos and failure.”

May also said the government is prepared to intervene in consumer markets to reduce costs for bill-payers. She has highlighted energy as one sector that she says is not working for consumers.

With May’s Conservatives 18 points ahead of Labour in polls, the prime minister said the opposition was out of touch with the concerns of the British people and had betrayed the Jewish community by only suspending a leftist former London mayor for saying Adolf Hitler had supported Zionism.

Under pressure from his own lawmakers for pulling Labour away from the center ground from where it won three elections under Tony Blair, Corbyn has also launched Labour’s local election campaign, saying the party was spurred by anger about the Conservatives “running our country down”.

“The future of our country cannot be left to the free market and the whims of the wealthy,” Corbyn said on Tuesday, promising to create a national investment bank and regional development banks to help unlock 500 billion pounds ($620 billion) for capital projects and to increase spending on education and health.

Pollsters say Labour could lose around 125 council seats across Britain in the elections, handing many to the Conservatives - a scenario not often seen as voters usually punish the ruling party in local elections.