76% of CWU members voted in favour of a national strike, the first in two years.

Dave Ward, CWU deputy general secretary, said: "This is a huge vote of no confidence in Royal Mail management. The company has tried to make out that problems only exist in some local offices, but postal workers across the UK have now spoken and they say no to Royal Mail's arrogance.

"Royal Mail has never really been engaged in modernisation. They've been running down the business, running down services and cutting costs and it's that business plan that postal workers have overwhelmingly rejected today."

The Royal Mail has branded the action "totally unjustified" and a "clear attempt to undermine the essential modernisation" of the postal service. It said that all current changes had been ratified by the CWU in 2007.

It pointed out that mail volumes are down by around 10% year-on-year as competition from email and the web accelerates. Each 1% decline in postal volumes currently costs the Royal Mail around £70m in lost revenues.

Royal Mail managing director Mark Higson said: "The ballot further underlines the CWU's determination to renege on the existing 2007 agreement on pay and modernisation, which the union's leadership signed in the presence of the Trade Union Congress.

"The CWU leadership is well aware that it has already agreed all the changes Royal Mail is making and we urge them to recognise the tough conditions faced by all our customers and Royal Mail itself."

The postal service is struggling to adopt to declining postal volumes and the need to modernise. It has a pension deficit estimated to be between £6bn and £10bn, which the CWU said the government has a "moral obligation" to fund.

Ward said today: "The Government must act now to resolve the pensions deficit which is crippling the Royal Mail's finances and chances to modernise effectively."

In a further blow to the postal service, reports today claimed that Amazon has cancelled its long-term contract with Royal Mail over fears that the strike action could disrupt Christmas deliveries. The contract, which covers parcels weighing more than 500g, is reported to be worth at least £25m a year, the Guardian claimed.

There is currently an estimated backlog of around 20m items in the postal system. Direct mail companies have previously expressed anger at the strikes, fearing that such action will force users to explore other media.

Lance Hill, managing director of print services, at direct mail house 4DM said that the company was "concerned" about the impact of the action.

"This is going to cause us production issues at our busiest time," he said. "The Royal Mail is taking positive steps towards mailing houses at the moment and we see it as a voice of the industry.

"However, ongoing strike action is undermining confidence in direct mail and the Royal Mail needs to act decisively to restore that confidence."