LONDON — IRISH terrorists appear to have changed tactics and are now concentrating on targets in mainland Britain, particularly London. That is the conclusion government security experts were drawing from bomb attacks Feb. 18 on two of London's biggest railway stations - one a ``warning'' dawn blast at Paddington Station, in which no one was injured. The other came during morning rush hour at Victoria Station, killing one person and injuring 43.
The blasts came 11 days after a mortar attack by Irish Republican Army (IRA) terrorists narrowly missed 10 Downing Street, the official residence of Prime Minister John Major, while his Cabinet was meeting there.
A senior police official said the strategy switch ``seems to take us back to the early 1970s, when the IRA launched numerous attacks on railway stations, department stores, and public houses.'' The most recent such attack, in 1983 on Harrods, the big central London department store, killed six people.
Soon after the explosion on a main platform at Victoria Station, George Churchill-Coleman, head of Scotland Yard's antiterrorist branch, accused the IRA of the bombing.
Later, claiming responsibility for the attack, an IRA spokesman said the ``blame'' lay with British authorities for failing to clear London's 11 mainline railway stations after a telephone warning 40 minutes before the Victoria blast.
Scotland Yard officers say the aim of the bombers was to cause maximum disruption. For most of the day, all London railway stations were shut down as more than half a million commuters milled about in confusion. Heathrow Airport was closed for two hours following a hoax bomb warning.
A large part of central London was closed to cars and other traffic as police responded to other hoax calls, many believed by Scotland Yard to have come from the IRA.
In an apparent attempt to limit public alarm, the government reacted calmly to the station attacks. Mr. Major resisted calls in Parliament for an official statement and instructed a junior minister to tell the House of Commons that a measured response would ``deny the terrorists the oxygen of publicity.''
But the government is coming under pressure to tighten up antiterrorist laws and improve police arrangements.
Ivor Stanbrook, a senior Conservative backbencher, called for a ``radical rethinking'' of the way antiterrorist intelligence is gathered. At present, eight different organizations are involved, and this is too cumbersome, Mr. Stanbrook said.
Since the IRA launched its previous terrorist campaign on the mainland 18 years ago, security experts say, the organization has been streamlined. A new generation of bombers is also at work. Terrorist units now operate as small ``cells,'' which may be placed in London or other cities for months or years before they are activated by the IRA high command.
The Republican News, an Irish newspaper that supports the terrorist cause, printed an interview last weekend with an IRA member. ``An action in Britain has many times the effect of a similar action in Northern Ireland,'' the IRA member said. ``We will not allow ourselves to be held to one tactic. We will spread the Crown forces into guarding as many areas as possible. Stretch them to the utmost. Nibble and bite them from every angle.''
Scotland Yard officers note that bombs and hoax calls aimed at disrupting traffic have been standard IRA tactics in Northern Ireland for many months.
A key concern of Scotland Yard and other antiterrorist officers is that the IRA may decide to turn its attention to the London Tube. Bombs or bomb threats against the subway system would pose a difficult problem. The system operates over 254 route miles and has 272 station, many of them deep under ground.