Two million public sector workers in Britain went on strike on Wednesday over changes to their pensions in the biggest walkout for decades which is expected to cause widespread disruption.

Thousands of schools were closed, hospitals were only ensuring emergency care, local authorities were paralysed and airports and ports were expected to be badly affected.

More than 1,000 demonstrations were expected to take place across Britain in scenes reminiscent of the 1970s.

Passengers arriving at London's Heathrow airport, one of the world's busiest air hubs, have been warned to expect delays of up to three hours to have their passports checked as border control officials join the action.

The strike will be the biggest test so far of Prime Minister David Cameron's Conservative-Liberal coalition government, which sparked the unions' fury by making public sector workers pay more into their pensions and work longer.

Anger rose further on Tuesday when finance minister George Osborne targeted the pay of teachers, nurses and soldiers and revealed plans to cut an extra 300,000 public sector jobs as he slashed Britain's growth forecasts.

Osborne also infuriated the unions by announcing a two-year, one-percent cap on public sector pay rises.

On Wednesday, Osborne said the strike would only harm the economy, and called for unions to return to the negotiating table.

"The strike is not going to achieve anything, it's not going to change anything," the Chancellor of the Exchequer told BBC TV. "It is only going to make our economy weaker and potentially cost jobs.

"So let's get back round the negotiating table, let's get a pension deal that is fair to the public sector, that gives decent pensions for many, many decades to come but which this county can also afford and our taxpayers can afford.

"That is what we should be doing today, not seeing these strikes." But Brendan Barber, general secretary of the Trades Union Congress (TUC), said the public sector was 'under attack' by the government and the strike was fully justified.

"There comes a time when people really have to stand up and make a stand," he told ITV.

"With the scale of change the government are trying to force through, making people work much, much longer and get much, much less, that's the call people have made."

Barber denied that public sector workers' pensions were 'gold-plated' compared to their counterparts in the private sector, and that the majority were low-paid workers.

At Heathrow Airport, immigration staff were drafted in from other sites to replace striking workers and some airlines cancelled flights in a bid to avoid long queues at immigration.

Passengers arriving on early morning flights from Australia and the United States reported few problems.

In central London, pickets were being formed around all the major public service buildings. Elsewhere in England, a giant union rally was to take place in the industrial central city of Birmingham.

In Scotland, 300,000 workers were expected to walk out and transport in Northern Ireland was facing disruption.

Under the government's proposals, public sector workers will be asked to work until they are 66 and increase their pension contribution payments.

The government also plans to peg pension payouts to the Retail Price Index rate of inflation rather than the higher Consumer Price Index measure as it seeks to address the dual challenge of increased life expectancy and a depleted public purse.

Staff also face a lower pension payout, based on their average salary as opposed to the final salary schemes to which they are currently tied.