The 28-nation Nato alliance should urgently undertake a "radical reform" to prepare for either eventuality, including by establishing a continuous presence of Nato troops in its "vulnerable" Baltic members Estonia, Latvia and Lithuania and pre-positioning military equipment in the three former Soviet states, a cross-party panel of MPs said.
"Dramatic" improvements should also be made to existing rapid reaction forces, headquarters structures should be established for eastern Europe and the Baltic and large-scale exercises involving military and political leaders from all Nato states should be conducted.
And they said that Nato should reconsider its Article 5 commitment for all members to come to the aid of any member which is attacked, to make clear that this includes countering unconventional threats such as cyber-attacks, information warfare and the use of irregular militia.
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Accompanied by the Duke of Edinburgh, the Queen will attend the opening ceremony of the fifth session in the Holyrood chamber.
Ms Leadsom has received the backing of Ian Duncan Smith, the former work and pensions secretary.