Berlin: The far-right Alternative for Germany (AfD) party on Monday formally cautioned one of its lawmakers over a tweet calling former tennis star Boris Becker's son a "little half-negro".

A tweet from the account of Jens Maier, a former judge who is now an MP, had attacked Noah Becker for reportedly complaining about being seen as the "eternal son" of his famous father.

"It seems the little half-negro simply got too little attention," read the tweet posted from the account of Maier, one of more than 90 AfD members elected to parliament last September.

Maier claims he did not write the tweet, which has since been deleted, saying one of his staff had posted it. In a statement issued Monday, Maier apologised to Becker for the "lapse" by his employee, who he said no longer worked for him.

"This tweet not only contradicts my style but also does not reflect my ideas. I will ensure that this does not happen again," he said.

Maier's explanation appeared to have been accepted by the party's leadership, which warned him to take greater care in managing his employees.

Noah Becker has filed a criminal complaint against Maier over the tweet, which was also blasted by his father in a scathing column in Sunday's edition of Die Welt newspaper. "Jens Maier says such things neither out of stupidity nor fear. He knows exactly what he is doing and why," wrote Boris, demanding that the MP face "consequences".

It was a second time in a week that AfD deputies had come under fire after police filed a complaint against senior party member Beatrix von Storch over a New Year's Eve tweet which they say violated laws against incitement to hate. Von Storch had criticised Cologne police for sending a New Year's greeting in Arabic on Twitter, asking if authorities had meant "to placate the barbaric, Muslim, gang-raping hordes of men?"

On January 1, a law against online hate speech went into effect in Germany, requiring social media companies to remove illegal inflammatory comments or face up to 50 million euros ($60 million) in fines. The AfD capitalised on discontent against a mass influx of asylum seekers to Germany since 2015 to make the strongest showing for a far-right party in a national election in the post-war era.

"[Noah] said that those who don't know the world, who haven't seen people with different skin colours or people who think differently, become scared," Boris wrote. "I have to admit he's one step ahead of me - I want there to be consequences first," the three-time Wimbledon champion added.

The tweet posted on Maier's account was also written in response to an interview Noah gave in which he described Berlin as "a white city" compared to other European capitals, and said he had been attacked there because of his skin colour.

The head of Germany's federal judges association, Jens Gnisa, condemned the tweet as completely unacceptable.

The AfD capitalized on discontent against a mass influx of asylum seekers to Germany since 2015 to make the strongest showing for a far-right party in a national election in the post-war era. Agencies