Angela Merkel delivered an emotional speech as the Christian Democrats prepare to choose a new leader, bringing her 18-year reign to an end.

Ms Merkel, 64, said in October she would leave her post as party chief but remain as chancellor of Germany until the end of the parliamentary term in 2021.

The politician gave an emotional speech as her party met to pick her successor in Hamburg on Friday.

Fighting back tears as she received a standing ovation, she told those gathered: "It has been a great pleasure for me, it has been an honour."

Ms Merkel, who said she would remain neutral on her successor as party chief, added: "In times like these, we will defend our liberal views, our way of life, both at home and abroad.

"The CDU in 2018 must not look back but look forward, with new people...but with the same values."

Three frontrunners are vying to replace her in a race that will put the winner in pole position to succeed her as the country's leader.

Sometimes dubbed ‘mini Merkel’ or AKK, Annegret Kramp-Karrenbauer, 56, is seen as the continuity candidate and is favoured by the CDU elite for her ability to unite the party and deliver election victories.

A former premier in the tiny western state of Saarland, she led a broad coalition there with the Greens and pro-business Free Democrats – demonstrating her ability to form alliances across Germany’s fractured political landscape.

Friedrich Merz, 63, is hoping to make a comeback to frontline politics after losing out to Ms Merkel in a power struggle in 2002 and leaving the Bundestag in 2009.

His socially conservative, pro-business message appeals to the CDU’s core of western, Catholic men who see Ms Merkel - a Protestant woman from the east - as an anomaly.

Annegret Kramp-Karrenbauer arrives for the CDU board meeting at the Hotel Atlantic Kempinski in Hamburg (EPA)

He is widely known for a proposal to simplify the annual tax return so that it could be written on a beer mat.

Jens Spahn, 38, has led criticism within the CDU of Ms Merkel’s 2015 decision to leave German borders open to more than one million refugees fleeing war in the Middle East.

Highly ambitious, he was awarded the Health Ministry portfolio after last year’s federal election.

He appeals to conservatives on the right of the CDU, but his chances appear to have been thwarted by Mr Merz’s comeback.

The new CDU leader will be chosen by 1,001 delegates who will vote at a party conference in Hamburg today, Friday.

To win, a candidate must secure more than 50 per cent of votes.

Friedrich Merz, member of the German Christian Democratic Party (AP/Michael Sohn)

If there is no winner after a first round of voting, a run-off will be held between the two candidates to win most votes.

The winner will likely lead the CDU in the next federal election due by October 2021.

A survey for broadcaster ARD on Thursday showed 47 per cent of CDU members favoured Ms Kramp-Karrenbauer compared with 37 per cent for Mr Merz and 12 per cent for Mr Spahn.

Economy minister Peter Altmaier, an ally of Ms Merkel, said: “I am convinced that with Annegret Kramp-Karrenbauer we have the best chance of the CDU winning an election,” adding she would be the most dangerous candidate to face the centre-left Social Democrats and the ecologist Greens

German Health Minister Jens Spahn arrives for the weekly cabinet meeting at the chancellery in Berlin. He is standing to replace longtime German chancellor Angela Merkel as Christian Democratic Union party leader. (AP/Markus Schreiber)

Ms Kramp-Karrenbauer has differentiated herself from Ms Merkel on social and foreign policy by voting in favour of quotas for women on corporate boards, opposed by Ms Merkel, and by taking a tougher line on Russia.

She has previously said Europe and the United States should consider blockading Russian ships over the Ukraine crisis.

But on what lies ahead for the CDU, Ms Kramp-Karrenbauer says: “I have no particular recipe.”

By contrast, Merz takes clear positions that appeal to rank-and-file party members hungry for a more clearly defined party after 13 years under Ms Merkel as chancellor.

He wants tax cuts, a stronger EU and a more robust approach to challenging the far-right.

Mr Merz will benefit from the fact that 296 of the delegates at the congress - almost a third - will be from his home state, the western region of North Rhine-Westphalia.

One senior CDU official, speaking on condition of anonymity, said many delegates were undecided before the congress and could be swayed by how the candidates present themselves on Friday.

“It could come down to the speeches on the day,” he said.