Douglas Alexander, the head of Labour’s election strategy, has said it is getting harder for politicians to campaign in elections because of conspiracy theories on social media.

Speaking at a conference in central London organised by LabourList, the shadow foreign secretary said that voters were increasingly getting their information from the “echo chamber” of Facebook and Twitter.

“We are used to a politics where we share facts, but diverge on opinion,” he said. “We are confronting increasingly, because of the rise of social media, a politics where people’s social media feeds can be an echo chamber for, at best, their own opinions and, at worst, their own prejudices.

“And that’s a tough challenge for all democratic politicians in every party of the UK, and more broadly.”

I said, ‘Do you mind if I ask where you get your news?’ And she said, ‘I get if off Facebook’ Douglas Alexander

To illustrate his point, he recalled a conversation with “an intelligent woman” in a supermarket in his constituency. She told him that she did not believe the results of the independence referendum, and that she thought there has been a conspiracy. She also thought the oil companies were involved in a global conspiracy to keep oil prices low, he said.

“I said: ‘Do you mind if I ask where you get your news?’ And she said, ‘I get if off Facebook every night’.”

He added: “How do we engage in a very rapidly changing media landscape in which facts are not common, actually people have their own fact[s]?”

After the Scottish independence referendum, a rumour spread on social media that there had been substantial vote rigging. An online petition calling for a recount received almost 100,000 signatures.