(Bloomberg) — Wes Streeting, in his first public appearance since resigning as Britain’s health secretary last week, announced he would take part in any leadership contest to replace Keir Starmer and called for Britain to rejoin the European Union.

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Streeting said that leaving the EU was “a catastrophic mistake,” and that Britain’s future lies within Europe and “one day, back in the European Union,” in his first speech since quitting Starmer’s government on May 14.

Speaking at a conference hosted by a Labour-aligned organization in London on Saturday, he said progressives in the country are “increasingly losing faith that Labour is capable of rising to the challenge,” reiterating that the party needs to see “a battle of ideas,” not personalities.

In response to a question following the speech, he said: “We need a proper contest with the best candidates on the field, and I’ll be standing.”

He called Labour’s disastrous showing in the recent local elections as “unprecedented,” and warned that nationalists are in power “in every corner of the UK.”

Streeting is only one of the party figures likely to throw their hats into the ring in the event of a formal leadership competition. His declaration brings the politically fraught issue of Brexit back in the spotlight at an awkward time for potential rival Andy Burnham, the Greater Manchester mayor who said on Thursday that he planned to contest the seat in Makerfield vacated by Josh Simons.

Simons resigned the same day to offer Burnham a chance to return to Westminster in a by-election scheduled for June 18.

Emerging victorious from the ballot is far from a foregone conclusion for Burnham. In Makerfield, in northwest England, around two-thirds of local voters backed Brexit in the 2016 referendum, and the constituency is hotly contested by Nigel Farage’s Reform party, which picked up 50.4% of the vote share in the May 7 local elections, with Labour taking just 22.7%.

While Burnham has previously signaled that he would support rejoining the EU, he struck a more cautious tone in an interview with ITV News on Saturday. “In the long-term there is a case for that, but I’m not advocating that in this by-election. In fact, what I am saying is focus now domestically,” he said.