Europeans leaders are becoming “more and more impatient” with the reluctance of west Balkan leaders to accept the necessity of reforms to the rule of law, threatening the chances of them joining the European Union by 2025.

The EU commissioner for enlargement, Johannes Hahn, told the summit for the west Balkans in London: “In all these countries I face a certain reluctance to address properly rule of law issues. It is a must for the EU.

“I admit our member states are becoming more and more impatient on this issue because they see this is as crucial – the fight against corruption, media reform and independence of judiciary.” He said statesmanship was needed in the region if the outline timetable for accession was to be met.

But he insisted the accession of countries such as Albania, Serbia, Macedonia, Montenegro and Kosovo into the EU represented a chance to export stability to the region, adding the alternative of the status quo will simply lead to the import of instability into the EU.

The London summit, part of the five-year-old Berlin process, is intended to help prepare the Balkan countries for EU membership by speeding economic reform, strengthening security and deepening the rule of law. Britain has also been promoting reconciliation by backing measures to locate and identify 12,000 bodies still missing from the civil war of the 90s, but hopes that a full-scale truth commission would be supported at the summit proved too ambitious.

The UK had intended by hosting a summit dedicated to EU enlargement that it could show that despite Brexit it remained committed to Europe, and its security.

The plan was torpedoed by the abrupt resignation of the foreign secretary Boris Johnson on Monday, the opening day of the summit, complaining in his resignation letter the UK was about to become a colony of the European Union.

Balkan leaders on Monday were left waiting for Johnson, their principal host, to decide whether to resign his post – a delay that required his deputy Sir Alan Duncan to chair the summit. Johnson’s successor, Jeremy Hunt, attended the second day of the summit, along with Theresa May, Angela Merkel and Federica Mogherini, the EU foreign policy chief.

The summit reviewed the mixed signals on whether the imminence of accession negotiations is speeding reform in the Balkans. The EU has said it will not allow into membership any state that has unresolved border disputes, a precondition that requires resolution of a host of complex ethnic, territorial and political disputes including between Bulgaria and Macedonia, Greece and Macedonia, and Kosovo and Serbia.

In a breakthrough, a 27-year dispute between Greece and Macedonia could soon be over – the latter’s official name has been resolved by two reformist leaders, with an agreement to rename it Northern Macedonia. But the package is facing resistance in Greece and awaits a tricky referendum in Macedonia this autumn.

Nikola Dimitrov, the Macedonian foreign minister and one of the most pro-reform figures in the Balkans, said: “The ultimate goal is to be a European democracy, but it is really high time our region stepped it up and took responsibility. We do not have the luxury of missing a golden opportunity.”

Referring to the decision by the EU last month to start accession talks with Macedonia and Albania next summer, he explained: “We have been locked in the waiting room for some time, but we are now running after a train, even if we are not quite sure in what direction.”

Asked why Macedonia wanted to join the EU at the time of the UK’s departure, Dimitrov said: “Perhaps those inside forget how cold it is outside.”