“I went into politics in the Scottish parliament in 1999. That’s 24 years. I’ve spent well over seven years with three phones by my bed,” he told The Times.
When asked what the devices were for, he replied: “Secret, secret and secret.”
He also said Kyiv might be wise to let its supporters “see gratitude”.
Prime Minister Rishi Sunak later pushed back against the comments, saying Ukraine’s President Volodymyr Zelenskyy had “expressed his gratitude for what we’ve done on a number of occasions”.
Mr Zelenskyy, speaking at the same event in Lithuania, also responded: “I believe that we were always grateful to the UK.”
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PM quizzed on Ukraine gratitude
Following the news of his departure on Saturday evening, Mr Wallace took to Twitter in an attempt to clarify his “Amazon” comments.
In a series of tweets posted in Ukrainian, he said his remarks were “somewhat misinterpreted”.
“I said that Ukraine sometimes needs to realise that in many countries and in some parliaments there is not such strong support as in Great Britain,” he wrote.
“It was a comment not about governments, but more about citizens and members of parliaments.”
He added that he meant to say Britain’s relationship with Ukraine is not “transactional” but more of a “partnership”.
Speculation about the defence secretary’s fate has been mounting for weeks, with officials inside the Ministry of Defence wondering who might replace him.
Mr Wallace had been facing the prospect of effectively losing his constituency of Wyre and Preston North at the next general election under boundary changes – meaning he would have needed to stand in another seat to remain as an MP.
The MP also revealed in his interview with The Times that, on the eve of the war in Ukraine, he discussed Britain supplying weapons to Kyiv – using whiskies as a code.
Referring to secret talks with his counterpart Oleksii Reznikov, he said: “the Nlaw [anti-tank missile] was Glenfiddich and Harpoon anti-ship missiles were Islay.
“I would text him saying ‘I’ve got some whisky for you’ or ‘the whisky is on its way’. We just picked codewords, minister to minister.”