Madeleine Moon, Bridgend's Labour MP, said more military exercises are needed to show Russia "we are ready and we are capable".
It follows the crash of Malaysia airliner MH17 in Ukraine and growing tensions between Russia and the west.
Ms Moon, who sits on the Commons defence committee, called for a halt in a decline on defence spending.
A total of 298 people were on board MH17, when it was reportedly hit by a missile over a rebel-held area in Donetsk region on Thursday.
Ukraine and the pro-Russian rebels have accused each other of shooting down the Boeing 777, which was flying from Amsterdam to Kuala Lumpur.
Western countries have criticised restrictions imposed by rebels at the crash site, and have asked Russia to pressurise them to allow more access.
Ms Moon said the "callous" behaviour of militia followed the annexation of Ukraine adding that Russian president Vladimir Putin had to realise that the "world is outraged".
She said: "I think we have to act as if we're entering a new Cold War.
"We've had Russian ships, we have had Russian submarines off our coast for the last few years.
"We've had Russian planes entering very close to our airspace for years, testing our defences.
"We've got no maritime patrol capability - that needs to be replaced."
Ms Moon, who is also part of the Nato Parliamentary Assembly, told BBC Radio Wales' Sunday Supplement Britain had ignored Russia's "flexing of its muscles".
She added: "We have to have more regular live exercises and snap exercises so that we are clear that we are sending a message to Russia that we are not sitting back - we are ready and we are capable - because that's the only message the Russians understand."

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Britain has to "act as if we're entering a new Cold War" with Russia, an MP has claimed.
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