The UK prime minister is to urge EU Nato members to spend more on defence, at an informal summit in Malta.
Theresa May will brief leaders on her talks in the US with Donald Trump, when he indicated his commitment to Nato.
The UK is one of the few alliance members to meet pledges to spend a minimum of 2% of GDP on defence.
Mrs May will also pledge continued UK help with Europe’s migrant crisis – a focus for the summit – and to be a “reliable partner” after Brexit.
She will argue that she wants a “new, positive and constructive” relationship with the EU after Brexit, Downing Street said.
She is also expected to stress that a strong EU is in the interests of the UK.