Russia did have some advance warning that an attack was going to take place but they “didn’t take it seriously”, an expert at the Royal United Services Institute (RUSI) has told Sky News.

Senior associate fellow at the institute, Mark Galeotti, said it was currently a “very fast-moving and fluid situation”.

“However you play it, this is something that Putin cannot ignore. It is the first invasion of the Russian territory since the Second World War,” he said.

“And for someone who had been claiming that everything was going according to plan in Ukraine, it clearly puts a line to that.”

Mr Galeotti added: “It seems Russia did have some advance warning that an attack was going to take place, but they thought it was going to be a short and minor raid.

“They clearly didn’t take it seriously.”

He said Ukraine was going to have “a variety of objectives” – military and political.

When it came to military objectives, Mr Galeotti said Ukraine “wanted to draw troops away from the other fronts”.

And when it came to political objectives, the expert said “it was not just a chance to humiliate Putin” but “also a chance to convey to Europe and the US – whose support Ukraine relies on – that the war is not yet over”.