Jean-Claude Juncker will step down as president of the European Commission in May, following elections to the European Parliament. 

The Luxembourger's successor must be agreed by leaders of European Union member states following the May elections, that will see anti-EU parties bidding to increase their minority in the legislature.

The parliament, which must approve the leaders' nominee, will push them to choose a Commission president from among the lead candidates of the main parties - the soc-called Spitzenkandidaten - despite many leaders insisting they should be free to nominate anyone.

Here is a look at the main contenders. 

Conservatives

Manfred Weber - An MEP for 14 years, the German has led the EPP - the biggest EU parliamentary group - since 2014. Declaring he will run on Wednesday, he seems confident of support from German Chancellor Angela Merkel despite his youth, at 46, and lack of government experience typical of Commission presidents.

Michel Barnier - Former French farm and foreign minister, at 67 he is four years older than the outgoing Juncker, a former Luxembourg premier, but has raised his profile by running the Brexit negotiations with London for the Commission. He may lack support from Paris, where centrist President Emmanuel Macron has made clear his opposition to the whole Spitzenkandidat system.

Manfred Weber speaking at the 2017 EPP congress in Malta. Photo: Reuters/Darrin Zammit Lupi

Alexander Stubb - The former Finnish prime minister could challenge Weber at an EPP nominating convention in Helsinki on Novembe 8. Stubb, 50, competes in "Iron Man" triathlons and is multilingual unlike Weber, who does not speak French, or Barnier, who rarely seems comfortable in English.

Many other names cited have included Merkel allies Peter Altmaier and Ursula Von Der Leyen and French IMF managing director Christine Lagarde - not to mention the wild card of Angela Merkel herself, who is now in her fourth term.

Socialists

Maros Sefcovic - A Moscow-educated Slovak diplomat who has worked in Brussels since 2004 and is Juncker's vice president for energy said in June he would run. He is 52.

Federica Mogherini - The 45-year-old was catapulted into the high-ranked Commission post of EU foreign policy chief in 2014. She could benefit from efforts to promote female candidates and a better left-right balance in Brussels but may struggle to get the necessary support from the new populist coalition in Rome.

Helle Thorning-Schmidt - Danish prime minister until 2015, at 51 she is perennially cited as a centre-left hope for senior EU roles but lacks backing from the ruling right in Copenhagen.

Federica Mogherini is likely to find more support in Brussels than in her native Italy. Photo: Reuters

Frans Timmermans - Juncker's Dutch deputy, 57, is a former foreign minister and passionate, multilingual advocate for the EU but his party's national eclipse counts against him.

Pierre Moscovici - Former French finance minister, 60, now EU economics commissioner, his party's national disarray is also a disadvantage, as is German wariness over his commitment to Berlin's vision of a euro zone of tight public finances.

Nadia Calvino - Long a senior Commission civil servant, at 50 she has the rare distinction for EU Socialists of being in government, having been named Madrid's economy minister in June.

Liberals

Guy Verhofstadt - Former Belgian prime minister who leads the liberals in the EU parliament, his age (65) and outspoken advocacy of much more powers for Brussels may limit his appeal.

Margrete Vestager - As a woman, aged 50 and with a star profile in Brussels from attacking tax avoidance and monopoly powers among U.S. multinationals like Google and Apple as the EU competition commissioner, the Danish former economy minister is widely talked about as a liberal who could win support beyond her party -- even if Denmark's ruling conservatives oppose her.

Cecilia Malmstrom - Another straight-talking, 50-year-old Scandinavian woman who has had a big role in Brussels' tussles with Washington, the EU trade commissioner and former Swedish Europe minister could tick similar boxes to Vestager.

Mark Rutte - Dutch prime minister for eight years, the 51-year-old may be tempted by a new job. He is solidly pro-EU but appeals to those who want its budgets and powers kept in check.

Xavier Bettel - In five years as Luxembourg prime minister, during which he notably married his male partner, the 45-year-old has built good relations with fellow national leaders. They might baulk at choosing another Luxemburger after Juncker but his friendship with the even younger Macron could be an asset. 

Cecilia Malmstrom is being mentioned as a potential candidate. Photo: Reuters