Members of the Public and Commercial Services union (PCS) have begun their 48-hour strike over changes to redundancy pay.

The union is expecting up to 270,000 civil servants working across the country - including at museums, courts, ports, airports, driving test centres and offices - to take part in the industrial action.

The union has the backing of a large number of MPs, with more than 150 signed up to an early-day motion expressing concern about the changes in terms and conditions, but has been criticised by Cabinet Office minister Tessa Jowell because a number of other civil service unions have agreed to the deal.

PCS members in London plan to march to Parliament Square on Tuesday lunchtime where the union's general secretary, Mark Serwotka, and speakers such as Labour MP John McDonnell, are to speak to a rally.

In a statement, McDonnell said the government had "severely under-estimated the strength of feeling among civil servants and the anger that's built up and led to this dispute" and called for the government to return to the negotiating table.

Serwotka also said that PCS membership had grown since the strike was announced last month and warned the government that there is "incredible anger" amongst civil servants over the reforms to redundancy compensation.

The union argues that the changes will "rob" some civil servants of up to a third of their entitlements, worth thousands of pounds, when they leave their jobs.

"The government is tearing up the contracts of low-paid civil and public servants whilst it claims it can do nothing about bankers' bonuses because of contractual obligations," he added.

Defending the reforms, Cabinet Office minister Tessa Jowell said that five of the six civil service unions had come to an agreement with the government following 18 months of negotiations and a number of changes to the original proposal.

While the PCS represents 270,000 civil servants, the other five unions represent around 100,000 staff between them.

Jowell said the consultation with unions had resulted in increased protection for lower-paid staff. "Those earning £30,000 or less - 80 per cent of all staff - will still get up to between two and three years' salary, while civil servants earning over £30,000 will have redundancy pay capped at two times salary."

She added: "This package brings the civil service more into line with the rest of the public sector and still offers more generous terms than much of the private sector."

The Cabinet Office has also criticised PCS for deciding to take industrial action because turnout for the strike ballot was 31.6 per cent, with 63.4 per cent of members voting for industrial action and 81.4 per cent voting for action short of a strike.

Jowell said the strike was "disappointing" because "less than one in five of their own members voted in favour of strike action, and that, overall, this figure represents only around 10 per cent of the total civil service workforce".

However, the union has insisted that the turnout is similar to that normally seen in union postal ballots, and not dissimilar to turnout in local elections.

A spokesman also argued that the present government was running the country on the basis of support from only 22 per cent of the electorate, with only 36 per cent of those who voted in the last election having voted for the government.

The union has also criticised the use of a command paper to introduce the compensation changes, meaning there was no parliamentary debate, scrutiny or vote of the reforms.