UK Prime Minister Boris Johnson has "expressed the UK’s grave concern about Turkey’s military operation in northern Syria" to President Recep Tayyip Erdogan.
The two leaders spoke during a call on Saturday evening, a Downing Street spokesperson said.
Johnson also said the incursion could further worsen the humanitarian situation in northern Syria "and undermine the progress made against Daesh".
Turkey launched its military incursion into Syria on Wednesday and argues it is necessary to create a "safe zone" along the border where it plans to relocate the more than three million refugees it is hosting.
It came after US President Donald Trump withdrew some of his country's troops who had been backing Kurdish forces in the fight against the so-called Islamic State (IS).
At least 30 civilians have so-far died as a result of Turkey's offensive in northern Syria, according to the UK-based Syrian Observatory for Human Rights.
It said 74 members of the Kurdish-led Syrian Democratic Forces (SDF) and 49 Syrian rebels aligned with Ankara had also been killed.
The observatory added eight Turkish soldiers had been killed but said Ankara had only acknowledged the death of five of them.
“The Prime Minister was clear that the UK cannot support Turkey’s military action," the spokesperson said. "He urged the President to end the operation and enter into dialogue, and said the UK and international partners stand ready to support negotiations towards a ceasefire."
“The Prime Minister underlined that Turkey is an important partner for the UK and a NATO ally. He recognised Turkey’s role at the forefront of the fight against Daesh and its generosity in supporting refugees who’ve fled the civil war in Syria," they added.
Earlier on Saturday, Germany announced it had banned arms exports to Turkey over the incursion.
“Against the backdrop of the Turkish military offensive ... the Federal Government will not issue any new permits for all military equipment that could be used by Turkey in Syria,” Foreign Minister Heiko Maas said, according to a spokeswoman.
The US and European Union have already warned Turkey of possible sanctions over the offensive.