Sir Michael Fallon said the Labour leader’s failure to back the Trident nuclear deterrent or commit to hitting Nato defence spending targets had been “noted” by Moscow.
The Defence Secretary also said Britain must be watchful for any Russian hacking to interfere with the election and said cyber defences had been put in place to protect the campaign.
Sir Michael made the forthright attack on Labour’s defence policy on a visit to Estonia to mark the activation of an 800-strong British battlegroup on Russia’s doorstep.
Nato has deployed thousands of troops to the Baltic states and Eastern Europe to try to reassure them against Russian aggression.
Sir Michael said: “Russia will be watching Labour’s feebleness. Jeremy Corbyn has not supported this deployment. He has questioned it.
“He has not made clear how they would finance our two per cent commitment to Nato and at every point he has voted against a stronger defence, including the renewal of Trident last July.
“Russia will be watching that, will have noted that feebleness and will be watching it throughout this campaign.
He went on: “Putin would certainly welcome feebler British defence and any undermining of our deterrent or our commitment to Nato two per cent defence spending, or any gutlessness in response to Russian aggression would certainly be welcome in Moscow.”