Kent Police and army bomb disposal experts were called to Manston Airport after the wartime relic was discovered and are continuing to assess the scene.
There is "no immediate threat to life" so the device will be detonated over the next few days, according to the Ministry of Defence.
Transport Secretary Chris Grayling - who has recently faced criticism over several mishaps - planned to use the airfield as a giant HGV holding bay to prevent traffic jams around Channel ferry ports if the UK leaves the EU without a deal.
There are fears that miles-long tailbacks are expected approaching the port of Dover in the event of a no-deal.
Congestion could be caused at the ports by customs checks being reintroduced on goods.
In January, a test of the site was branded pointless and a "farce" after just 89 lorries took part.
Officials said the numbers involved were "sufficient" and insisted the test run from Manston to Dover was a success.
The plan is part of Operation Brock, which is designed to keep traffic flowing through Kent in case of cross-Channel delays.
The operation will also see a section of the M20 become a contra-flow system on one side of the carriageway, with lorries queuing on the opposite carriageway.