Andy Burnham will pledge to fight nationalist politics which is leaving the world "fragmented, divided and less secure" in a thinly veiled attack on his Labour leadership rival Jeremy Corbyn.
Mr Burnham will criticise those who question the membership of international alliances and institutions in favour of "isolationism" in an apparent slight to Mr Corbyn, who has called into question Britain's membership of Nato.
Mr Corbyn, the surprise favourite in the leadership contest, last week reversed his call for the UK to leave Nato, admitting there is no public "appetite" to pull out, but remains an outspoken critic of the alliance.
The Islington North MP has also refused to rule out campaigning for Britain to leave the European Union in the upcoming in/out referendum on the country's membership.
In contrast, Mr Burnham will pledge to "defend the UK at all costs, its place in Europe and its membership of international organisations".
In a speech at the Royal United Services Institute on Wednesday, he will say: "Across Europe, we have seen the rise of nationalist parties that lean to the left and to the right. And established parties have begun to follow suit. Here in this country, the governing party spent an election playing to English nationalism and openly stoked resentment of Scottish nationalism.
"So as nationalist parties take up positions of power, membership of international alliances and institutions will be questioned, as isolationist and protectionist policies are pursued. This risks leaving the world more fragmented, more divided and less secure than at any point since the end of the Second World War.
"As Labour leader, I will fight nationalism wherever I find it. It is a dead-end towards division, separation and conflict. I consider myself British, before I'm English, and an internationalist at heart.
"Labour has been too weak in standing up to nationalism. Those who believe in Britain, and believe in Britain in Europe, need to stand up and be counted now. That's what I will do. I will defend the UK at all costs, its place in Europe and its membership of international organisations."