David Cameron is set to broaden his assault on Labour's "waste" by pledging to cut billions from the welfare bill.

The Tories are to accuse the Government of squandering £30 billion on fraud and error in the benefits system since 1997.

They will promise tougher punishments for fraudulent claimants, which could see them lose rights to benefits for up to three years after "three strikes".

Meanwhile, Labour strategists believe that they can unpick the policy and win the on-going argument on National Insurance, despite a slew of senior business figures coming out against the planned rises.

In a speech later, Gordon Brown is expected step up his attack on Tory claims that £6 billion of efficiency savings can fund reversing the bulk of the planned increases.

The Tories' adviser Sir Peter Gershon disclosed that his proposed efficiency savings would involve slashing £2 billion from the public sector payroll, which experts estimate could involve a reduction of 20-40,000 jobs.

Mr Cameron declined to put a figure on the job losses, but said that the savings could be made by not filling vacancies when they arise, rather than by imposing redundancies.

The news comes as the Transport Secretary called on Liberal Democrat voters to back Labour to keep David Cameron out of power.

Lord Adonis, a former Lib Dem councillor, is calling for tactical voting in Labour-Tory marginals, where he says "a vote for the Lib Dems is a vote which helps the Tories against progressive policies".

Day four of campaigning is expected to see the Lib Dems focusing on bank charges on customers' accounts.

Leader Nick Clegg said banks should not be allowed to "profiteer" from people making small mistakes, as his party pledge to stop excessive penalties.

The party's manifesto for consumers will also contain measures to limit charges for bouncing a cheque and cap the interest rates on credit cards.