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By Chris Mason
Political editor, BBC News
The PM is travelling to Poland on Tuesday to meet Polish Prime Minister Donald Tusk and the secretary general of the Nato defence alliance - and promise more money to support Ukraine.
The UK will provide an additional £500m to Kiev on top of the £2.5bn allocated for this financial year.
Rishi Sunak wants to emphasise the UK's role in defending Ukraine.
Mr Sunak will then head to Berlin to meet the German Chancellor Olaf Scholz on Wednesday.
Europe confronts an "expansionist Russia" and Ukraine faces a "difficult summer", Downing Street says.
"Defending Ukraine against Russia's brutal ambitions is vital for our security and for all of Europe.
"If Putin is allowed to succeed in this war of aggression, he will not stop at the Polish border," Mr Sunak said.
Ukraine and Poland share a 300 mile border.
Poland also has a 130 mile land frontier with Russia itself - with Kaliningrad, the chunk of Russia, its exclave on the Baltic Sea.
The focus from Mr Sunak is the extra support for Ukraine and so he will argue for Europe as a whole: more ammunition, air defences, drones and engineering support.
More than 1,600 missiles are being provided, 162 armoured vehicles and 78 all terrain vehicles.
As well as meeting Mr Tusk, who used to be one of the European Union's most senior figures, and Nato chief Jens Stoltenberg, the UK will offer to deploy an RAF squadron to carry out policing of Poland's skies for Nato next year.
It is another reminder that alongside war in the Middle East, war continues in Europe too.
And conversation about it will continue on Wednesday in Berlin, when Mr Sunak meets the German Chancellor Olaf Scholz.
The US House of Representatives has finally approved billions of dollars in new US military aid for Ukraine.
The much-delayed measure had vocal opponents in Congress and it took a bipartisan deal to get the $61bn (£49bn) package through. It will now go to the Senate, which is expected to pass it within the next few days before President Joe Biden signs it into law.
And following a Nato-Ukraine summit last week, Mr Stoltenberg said Nato will give Ukraine more advanced air defences after urgent Kyiv pleas and deadly Russian attacks.