Europe faces a moment of reckoning but David Cameron has burnt bridges on the continent instead of building alliances, leaving Britain sleepwalking towards the exit, shadow foreign secretary Douglas Alexander has said.

He said the Prime Minister's pandering to Eurosceptic Tory backbenchers means he has not formed the relationships with other European Union leaders he needs to get what he wants in any renegotiated settlement. Labour does want to see a reformed EU but leaving it will be bad politics as well as disastrous economics, he said. Mr Alexander told the Labour Party conference in Manchester: "Europe does face a moment of reckoning which is why Labour has supported European sanctions against Russia and why, as Europe, we must stand united in our commitment today to Ukraine.

"And yet our Prime Minister has spent the past four years burning bridges instead of building alliances with other European countries. As a Tory leader, he has settled for following his backbenchers instead of leading his party or speaking up for Britain. Britain leaving Europe would present the biggest threat to British national prosperity in a generation.

"And as Labour we understand that Europe is both a strategic and an economic asset for Britain. Like British business, we understand that sleepwalking towards exit is not just bad politics, it is disastrous economics for our country.

"As Labour, now with 20 Labour members of the European Parliament, we want to see Britain at the heart of a changed European Union."