Cracks between the two emerged after the diplomat's public criticism of Ukraine's president days ago.
Volodymyr Zelenskyy on Friday dismissed Ukraine's ambassador to the United Kingdom, Vadim Pristaiko.
Pristaiko was a key figure in relations between Kyiv and London, one of its primary European allies.
In an interview on Sky News days before he was sacked, Pristaiko criticised Zelenskyy for his "sarcasm", while discussing contentious comments by UK defence secretary Ben Wallace.
The outgoing British official said people wanted Kyiv to show more gratitude for the massive amounts of support given by allies, during a NATO summit in Vilnius.
He quipped the UK is not "an Amazon" delivery service for weapons for Ukraine.
In response, Zelesnkyy later said: "How else can we show our gratitude? We can wake up in the morning and thank the minister. Let him write to me and tell me how to thank him."
When he appeared on Sky, former ambassador Pristaiko called Zelenskyy's response "a little bit of sarcasm", creating a rift between the pair.
"I don't believe that this sarcasm is healthy," he continued. "We don't have to show the Russians that we have something between us."
"They have to know that we are working together. If an issue happens, Ben [Wallace] can call me and tell me everything he wants."
Though the initial communique of the sacking did not give a reason, a diplomatic source has confirmed to the PA news agency that Pristaiko was given the boot over this public criticism.
Zelenskyy appointed Prisaiko in 2022. The 53-year-old - born and educated in the Odesa region - was formerly the Ukrainian minister of foreign affairs.
He has held a number of other senior diplomatic posts, including ambassador to Canada and head of Ukraine's NATO mission.