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The military exercises came amid fresh political wrangling in Britain over how to respond to Russia’s renewed assertiveness. Ken Clarke, the former Conservative Cabinet minister, attacked as “pointless” Mr. Cameron’s plan to send 75 British troops to provide logistical advice to Ukraine’s beleaguered forces.

“Military conflict doesn’t help,” he said. “I don’t have strong feelings about retraining the Ukrainian army, but it’s not going to solve anything because no matter how well trained and equipped the Ukrainian army are, the Red Army [Russians] could defeat them by the end of this week if they wanted to.”

Meanwhile, Rory Stewart, the chairman of the Commons defence select committee, said it would be a “big mistake” for spending on Britain’s military to fall below the NATO target of 2% of the national budget.

The Tory MP and former diplomat warned that Russia’s intervention in eastern Ukraine showed that spending needed to be maintained as a “symbolic” message to President Vladimir Putin.

George Osborne, the Chancellor, is believed to have told Mr. Cameron that spending was on course to fall below the target within two years and that he was content for it to do so.

Mr. Stewart told BBC radio: “The view of the defence committee is that that would be a big mistake, because that commitment came out of a Nato summit that was directed against what’s happening in the Ukraine.”

Meanwhile, a Russian newspaper claimed to have found a secret strategy document that advised the Kremlin to break up Ukraine and absorb its pro-Russian regions even before the country’s president fled following pro-European street protests a year ago.