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US President Barack Obama and his Russian counterpart Dmitry Medvedev have finalised a landmark deal to cut long-range nuclear arms, slashing the number of deployed warheads by a third.

Mr Obama hailed the treaty as the most comprehensive weapons control agreement in nearly two decades, saying that it sends "a clear signal that we intend to lead" on the issue of nuclear proliferation.

Tonight, Peter Marshall reports on how significant the foreign policy breakthrough is for Mr Obama, who set out his vision of moving towards a world without nuclear weapons in a keynote speech in Prague almost a year ago, and how likely it is that countries like Iran and North Korea will follow his lead.

We will also be talking to the US State Department.

Michael Crick will be looking at the Conservatives' economic policies, and how, in the wake of the Budget, the party intends to put clear blue water between its plans and those of the government.

For all that and more join Gavin Esler at 10.30pm on BBC Two.