The funding will be invested in advanced technologies including fitting Royal Navy ships with state-of the art directed energy weapons.
Ms Reeves is under intense pressure to get the economy growing amid a bleak backdrop of gloomy forecasts, including from the Bank of England.
The Treasury’s Office for Budget Responsibility watchdog is expected to follow the bank's example by halving economic forecasts for this year.
Despite the defence announcement, the Chancellor is bracing to wield the axe on spending at the fiscal event, with £15billion of cuts expected as she tries to balance the books.
The Government unveiled £2billion in grant funding yesterday to deliver up to 18,000 new homes in England to cool Labour anger at "austerity" moves to slash benefits.
It comes after stalling growth and rising debt interest costs - plus looming fears of a Donald Trump-instigated trade war wreaked havoc with Ms Reeves' plans following her controversial autumn Budget.
She must fill a hole in the public finances that could be as much as £15billion. It will have to come from lower spending as Ms Reeves has ruled out tax rises at this stage. The Chancellor, who visited a military vehicle maker in Telfo...