The foreign affairs problems a new prime minister will face

The success of nationalist parties in the European Parliament elections may complicate attempts by a UK prime minister trying to navigate a new relationship with the EU. Some leaders potentially supportive of the UK might be distracted by their own efforts to stay in power. Others might be less willing to do deals with the UK, say, on migration.

Labour has said it would want to negotiate a new defence and security pact with the EU, in a bid to formalise co-operation over issues such as the war in Ukraine, the threat from Russia, shared defence procurement, mass migration and organised crime.

But the new European Commission will not be formed until the autumn, so some in Labour fear there would be a political vacuum until then during which if they were in government, they would want to demonstrate its pro-European credentials without giving away its negotiating hand.

Labour has promised it will not take Britain back into the Single Market or Customs Union, let alone the EU (in contrast to the Lib Dems who want the UK to re-join the Single Market and the SNP which believes EU membership is the “best option for Scotland”.

Over time, European diplomats say, a Labour government might have to choose how much it is willing to align Britain’s economy to the EU’s in return for greater co-operation, on security via the new pact, or on trade via the scheduled review of the Brexit deal in 2025. This would inevitably be politically contentious because the government would have to find a balance between making economic gains and accepting EU rules over which it had no say.

The Conservatives promise to “build” on the Brexit deal they negotiated and keep removing EU laws from the UK statute book. Reform would scrap EU laws, abandon the Windsor Framework establishing Northern Ireland’s trading relationship with the EU and take the UK out of the European Convention on Human Rights.