As others have already pointed out...

From the BBC:

Mr Brown said the National Insurance rise would protect investment in schools, police and NHS guarantees, while the Conservatives would "put hospitals, police and our health service at risk" - claiming they would take £6bn out of the economy.

Others have already debunked the Big Fat Myth that tax cuts would 'take money out of the economy', see The Fat Bigot (long version), Adam Smith Institute (technical version) or The Crown (short version), [UPDATE: or John Page (medium length version)], but what is also worth highlighting is that:

* £6 billion is a huge amount of money but amounts to only about one per cent of total government expenditure, or one-thirtieth of our current annual deficit or indeed one-thirtieth of planned spending on NHS and the state education system (which is of course a lot more than the amounts actually spent on 'healthcare' or 'education' in their own right).

* On a static basis, additional receipts of £6 billion look 'about right' (i.e. new rate/old rate - 1 x total NI revenues x 3/4 of workers in private sector), but the Tories can't be bothered to point out that this will reduce turnover and profits chargeable to corporation tax by £6 billion as well, so the overall increase in revenues will be barely more than £3 billion. That's the 'Laffer Rainbow' in action - you can't just look at one tax in isolation. Sure, most of the £6 billion we first thought of gets spent again, but not necessarily on value-adding stuff, so let's ignore that effect.

* On a dynamic basis, things are even worse. Labour's plan is to increase National Insurance by two per cent from April 2011 onwards, i.e. Employees' contributions up from 11% to 12% and Employers' contributions up from 12.8% to 13.8% (none of this is final of course).

Assuming no changes to basic rate of tax or main rate of VAT, this would increase the marginal overall rate of tax for a basic rate taxpayer/employee from 38.8% to 40.2% (non-VAT-able business) or from 47.9% to 49.1% (VAT-able business), which is getting perilously close to the top of the Laffer Curve, so the chances are the increase in revenues will be considerably less than £3 billion and possibly negligible (or even negative).

Just sayin', is all.