Officials analysed the cost of the policy as part of the government’s dossier on opposition spending plans.
Labour said the analysis of one of its key NHS policies was ‘completely inaccurate and deliberately misleading’.
The party slammed a ‘dodgy dossier riddled with untruths’ unveiled by chancellor George Osborne on Monday as the two main parties unofficially launched their general election campaigns.
£100m Labour pledge
In May Labour leader Ed Miliband pledged £100m to fund a guarantee that all NHS patients would have access to a consultation or appointment if needed on the same day, a guaranteed appointment within 48 hours, or an appointment more than 48 hours ahead with a doctor of their choice.
Shadow health secretary Andy Burnham later suggested the guarantee could be written into the GP contract.
GP leaders attacked the plans as electioneering and said the £100m was not enough.
GPC chairman Dr Chaand Nagpaul said the 48-hour ‘target’ would ‘breed perverse behaviour and undermine GPs seeing patients on clinical need. We need capacity not targets.
He added on Twitter: ‘£100m less than 0.1% NHS budget. Call that investment or valuing general practice?’
The Treasury’s analysis calculated that the additional appointments required would cost £1bn for the 48-hour pledge, £12m for the same-day pledge, and £292m for the GP of your choice pledge, a total of £1.3bn.
Labour has said it would fund the scheme with £100m a year to fund an additional 300m appointments.
The money would come from savings from scrapping NHS competition requirements and cutting the bureaucracy of Monitor, the NHS Trust Development Authority and CSUs.
Labour has also pledged to use part of a £2.5bn fund from a mansion tax and a tobacco profit levy to pay for 8,000 more GPs.
On Sunday the opposition said the NHS would be unrecognisable after another five years of Conservative government with over 20m people waiting a week or more for a GP appointment, or unable to get one at all.
The Treasury document acknowledges, however, that the costing does not take account of potential savings the policy could create elsewhere in the system.
The Conservatives have said they would guarantee 8am to 8pm and weekend GP access for everyone in England by 2020 if they form the next government.