Struggling post offices could be about to lose 30 per cent of their income, the Liberal Democrats have warned.

The party claimed that next week's introduction of electronic payment of pensions and benefits will hit Post Offices and leave a question mark over the future of the branch network.

Figures from the Cabinet Office's performance and innovation unit concluded that paying out pensions and benefits provides the network with up to 40 per cent of its income.

Ministers had calculated that around three million people would opt to have their money paid into Post Office card accounts which would cushion the blow.

However the government admitted the take-up has been much lower than anticipated and the final total could be 2.4 million accounts.

The party also warned that the government is set to add to the crisis by cutting the fees it pays the Post Office for transactions.

Vince Cable, the Liberal Democrat DTI spokesman, said ministers had heaped problems on a struggling organisation.

"The government has hit Post Offices with a double whammy - no new business and the new Post Office account is so bureaucratic that existing customers are being driven away," he said.