British transport workers' unions called the first national rail strike in 16 years on Thursday, causing a new headache for the Labour government weeks before an election.

The four-day walkout over planned job cuts and changes to working practices at Network Rail, operator of the rail infrastructure, will start on April 6, just after the long Easter weekend which unions said they did not wish to disrupt.

In a joint news conference, the RMT and TSSA unions said the planned changes by Network Rail would compromise passenger safety. However, they left a door open for further negotiations with Network Rail management to avert the walkouts.

"We do not want to spoil the Easter holidays .. This still gives us an opportunity of nine days of trying to resolve the dispute," said Bob Crow, general secretary of the RMT union.

The unions said maintenance workers would strike from the morning of April 6 to the late evening of April 9, while signallers would stage two four-hour stoppages a day, during peak hours, for four days starting from April 6.

Network Rail denied that the proposed 1,300 job cuts and changes to staff rosters would have any impact on safety.

"The issue of safety is a smokescreen from a union leadership stuck in the steam age," it said in a statement.

"Our contingency plans are well advanced and aim to keep as many trains running as possible. But a national rail strike will have a severe impact on services and on Britain," it added.

PLANES AND TRAINS

The rail dispute comes in the middle of a separate crisis at British Airways (BA), where cabin crew staged a three-day strike from March 20 and are planning a further four-day stoppage from March 27. The BA walkouts have disrupted Easter travel plans for tens of thousands of families.

The disputes have created a rash of negative headlines and talk of a "spring of discontent" ahead of a national election expected on May 6. The Labour Party, which has strong links with unions, trails the opposition Conservatives in the polls.

Prime Minister Gordon Brown's spokesman said strikes were in nobody's interest and Brown hoped talks between the unions and Network Rail would continue and the dispute would be resolved.

The Conservatives' transport spokeswoman, Theresa Villiers, sought to use the strikes to land a blow on Brown.

"Militant unions are cashing in on Gordon Brown's weak government and threatening millions of passengers on our airplanes and railways in the run-up to Easter," she said.

The Labour Party receives most of its funding from unions, which has added embarrassment for Brown ahead of the election.

The Conservatives have sought to revive memories of a wave of strikes during the so-called "winter of discontent" in 1978-79, when Labour was in power.

Asked whether the rail strike risked playing into the Conservatives' hands and helping end Labour's 13-year stint in government, the RMT's Crow reacted angrily.

"This dispute is about Network Rail. It's nothing to do with the Labour government and it's nothing to do with the Tory (Conservative) Party," he said.

(Additional reporting by Mohammed Abbas, Adrian Croft and Avril Ormsby, editing by Adrian Croft)