The head boss at Blackberry is outraged after US T-Mobile offered his customers a free upgrade to an iPhone 5s. 

T-Mobile sent out the emails last week offering a free upgrade to an iPhone 5s, calling the deal a "great offer for Blackberry customers."

Blackberry customers started complaining over Facebook and Twitter, which led to Blackberry boss, John Chen, calling the offer "inappropriate and ill-conceived."

T-Mobile apologised, saying it was still happy to be a Blackberry partner.

Recently, Blackberry has been losing market shares to the iPhone, which, let's face it, can't have helped calm Mr Chen down.

John Legere, T-Mobile's US chief executive, tried to downplay the fuss, telling customers that T-Mobile were simply offering them a choice. 

However, a fuming Mr Chen wasn't having any of it.  In a blog on the Blackberry website addressed to his customers and the mobile company, he wrote, angrily:

"I can assure you that we are outraged too.  What puzzles me more is that T-Mobile did not speak with us before or after they launched this clearly inappropriate and ill-conceived marketing promotion.   I would like to remind T-Mobile that our long-standing partnership was once productive and profitable for both Blackberry and T-Mobile.  I hope we can find a way forward that allows us to serve our shared customers once again."

Once a main player in the smartphone industry, Blackberry have seen a recent decline in the wave of Apple and Samsung smartphones, and even its new launch of the Z10 had disappointing sales figures.