Labour activists knocking on doors in the run-up to the May 1 local elections will be armed with Gordon Brown’s latest weapon against the Tories — 10 billion pounds of tax cuts.

No — Brown hasn’t become the slash and burn premier. It’s the amount that Labour calculates Conservative tax pledges will cost — and Brown says that means 10 billion pounds of cuts in public services.

“They (the Conservatives) used not to tell us their manifesto until a day or two before the election,” Brown told party members at Labour’s Spring Conference in Birmingham on Saturday.

“We now know what they are standing for. Ten billion of tax cuts paid for by 10 billion of spending cuts. And in every council area in the country over the next few weeks, let us expose what they are doing.”

Brown said the Conservatives had a duty to say where the axe would fall — would it be on education, housing, social services, transport or the environment?

Brown didn’t give a breakdown of the 10 billion, so I asked party officials for the details. Here’s how they add up the Tory tax plans:

Reducing Corporation Tax to 27 percent — 1.75 billion
Transferable tax allowance for married couples — 3.2 billion
Increases in Working Tax Credit — 3 billion
Raising Inheritance Tax threshold to 1 million pounds — 2 billion
Removing Stamp Duty on houses for first time buyers — 0.4 billion

Unsurprisingly, the Conservatives dispute Labour’s sums — which actually add up to 10.35 billion pounds.

A Conservative party spokesman said Shadow Chancellor George Osborne remains committed to not making any unfunded tax cuts — so that revenue cut in one place would be covered by additional income elsewhere.

It was time to go back to the Labour officials — and they responded that the Conservatives haven’t properly identified the additional income and that therefore the tax cuts aren’t funded.

Follow the money … it’s an argument we can be sure will run and run.