Boris Johnson raised eyebrows on Friday when he greeted Poland's populist prime minister by grabbing his thumb.
One Downing Street photographer to witnessed the incident described the greeting as "very odd".
Mateusz Morawiecki was visiting Downing Street for talks while his country grappled with the migrantion crisis on its eastern border.
Mr Johnson told the right-wing populist that the UK would stand "shoulder to shoulder" with Poland against those who would "try to provoke a migrant crisis" on its borders.
But as snappers photographed the two leaders Mr Johnson grabbed his counterpart's thumb – leaving the Polish PM looking confused.
"Britain's Prime Minister Boris Johnson greets Poland's Prime Minister Mateusz Morawiecki on Downing Street, and grabs his thumb !!!! very odd," Steve Back said in a post on social media.
According to a read-out of the pair's meeting, the UK prime minister told Mr Morawiecki: "I think this is an important occasion - and we are very, very grateful to you for visiting us, for coming to Number 10, to London, because this is a moment where we can reaffirm our commitment to the relationship but also to standing shoulder to shoulder with Poland against those who would try to provoke a migrant crisis, for instance, on Polish borders."
Mr Morawiecki said that "things are now getting very much crazy around us".
The authoritarian government of Poland's neighbour Belarus has been accused of bussing in refugees and migrants from the Middle East and encouraging them to cross Poland's border.
The approach has been referred to as "hybrid warfare", amid claims it is intended to destabilise the European Union.
It comes as the UK grapples the arrival of thousands of people on its shores. Many people fleeing conflict and persecution are crossing the English Channel on small boats to try and claim asylum.
Poland’s government has been accused by the European Commission and human rights groups of undermining the rule of law with changes to the way its judiciary works.
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