A Munich-based business has issued a grovelling apology to Philipp Lahm over a spoof April 1 story claiming he has invested in them with Germanys ex-captain reportedly threatening legal action.

On Saturday, April 1, several German media outlets ran what later emerged to be a spoof story claiming Lahm, who will retire in June, has invested in the fitness start-up after the company newsletter carried a prank article.

The piece even contained a fake quote attributed to Bayern Munich star Lahm and an employee decided to circulate it to the media without the knowledge of the company's co-founder Vahid Imani.

But the joke quickly backfired as Lahm's management failed to see the funny side and have threatened to take the matter to court.

"This thing has nothing to do with fun. In our eyes, it is a cheap trick to use a celebratory for advertising," Lahm's advisor Roman Grill told German daily Bild, who had also briefly carried the spoof story.

The start-up firm say they have repeatedly apologised to Lahm's management, but the spoof has already generated lots of free publicity.

"I am very sorry that we triggered such a wave with the 'announcement'," said Imani in a statement.

"The first of April no longer seems to be what it once was.

"We, as sportsmen, admit our mistake and will make offerings on how we can rectify things, like by making a donation to the Philipp Lahm Foundation."

Imani says he actually met Lahm at a Bayern Munich function, gave him a business card and spoke to Germany's World Cup winning skipper about the start-up, but nothing more came of it.