Royal Mail is heading for a bitter confrontation with postal workers after announcing plans to recruit an army of 30,000 temporary staff in an attempt to crush the national strike that starts this week.

In a move that stunned union leaders and raised tensions between management and workers to new levels, Royal Mail said it had ordered the biggest recruitment drive in its history "to help keep the mail moving during the strikes called by the Communication Workers Union (CWU)". Sources inside the CWU, which has called national strikes for Thursday and Friday, questioned whether the move was legal and suggested that it could be challenged in the courts.

As householders and businesses braced themselves for massive disruption, business secretary Lord Mandelson said he was "beyond anger" with the union for obstructing change and modernisation. "This is a matter of life and death for the future of the Royal Mail," Mandelson told the Observer. "This national strike will drive away yet further customers and further business, possibly never to return to the Royal Mail."

Fears are now growing that the strikes could unleash some of the worst industrial strife since the miners' strike of 1984-85 with the focus being on possible violence at picket lines.

The Association of Chief Police Officers (Acpo) said that it was closely monitoring the situation and had issued guidance to forces on dealing with large-scale strike action. Each police force is assessing and reviewing the implications for public disorder that might arise from industrial action. An Acpo spokesman said: "It is important that we keep the public safe as well as always preserving the right to protest."

On Thursday CWU workers at mail distribution centres across the country will come out on strike. Royal Mail insists there will still be collections and deliveries. On Friday, however, those who actually collect letters and parcels from postboxes and deliver them will strike, meaning there were will be no service to households and businesses across much of the country. Royal Mail is also suspending its "next-day delivery" guarantee, under which customers pay extra to ensure letters and parcels arrive before 1pm on the next working day.

The company normally employs an additional 15,000 staff in the run-up to Christmas. But tonight it said that it had decided to double that number "to help offset the impact on customers of the CWU's unjustified and irresponsible strikes".

Those employed on temporary contracts were not being brought in "to do our postmen's work when they are out on strike, but to make sure that we have people to help clear any backlogs between strikes, as well as to help, as happens every year, with the seasonal build-up of mail in the run-up to Christmas," the company said. It added that the move was "fully in line with all employment law".

Royal Mail group chief executive Adam Crozier said: "We are continuing to urge the union to halt its appalling and unjustified attack on customers. At the same time, we are absolutely determined to do everything we can to minimise delays to customers' mail.

"Every year, Royal Mail recruits thousands of additional, fully vetted, temporary staff as part of the operation that successfully delivers the Christmas mail. This year we'll have twice as many people on board, and we'll have them in place much earlier in the autumn."

A CWU spokesperson said: "We're disappointed that Royal Mail appears to be more interested in sidelining the views and concerns of its staff than reaching an agreement to bring this dispute to an end.

"Instead of spending vast sums of money on untrained temporary workers we urge the Royal Mail to engage with talks to reach an agreement to get the permanent staff back to work. Royal Mail is planning for failure here instead of addressing the concerns of its staff. Postal workers deserve more than this dismissive attitude. CWU remains available for talks to avoid a strike."

The union maintains that Royal Mail has tried to force through modernisation and changes to working patterns without proper consultation. The strikes are expected to cause huge problems for both business and domestic customers. There are particular concerns within the NHS that letters due to go out shortly to people regarded as at risk from swine flu, and who would need vaccination, could be held up.

In an interview with the Observer, Dave Ward, deputy general secretary of the CWU, also claimed that Royal Mail was planning to scrap 60,000 jobs in the next two years.

Postal workers were in a "battle over the future of the industry... It [the dispute] is on a scale which comes along only every 50 or 60 years," he warned.

He added that state-owned Royal Mail was the last major industry in Britain not to have undergone a radical restructuring. But he stressed that the CWU was keen to find a solution to the dispute. Ward accepted "there is going to be pain along the way" for its members, adding: "We are determined but we desperately want to find a solution. I would not suggest this [dispute] is like the miners' strike in terms of bitterness."

Royal Mail admitted that modernisation plans would inevitably lead to job losses but insisted this weekend that it would abide by its policy of implementing only voluntary, and not compulsory, redundancies. It rejected the CWU's estimate of 60,000 jobs lost, but could not specify how many would be lost.

Larry Whitty, a former general secretary of the Labour party and now chairman of Consumer Focus, said: "If Royal Mail and the CWU are unable to engage in a positive manner, then it seems appropriate to us that mediation is a necessity and not a luxury."