The Soviet Union expelled 25 British diplomats, journalists and businessmen from Moscow today in retaliation for Britain's expulsion Thursday of an identical number of Soviets whom the British accused of espionage. Westerners regarded the retaliation as unusually harsh.

Brian Cartledge, Britain's new ambassador to Moscow, was summoned this afternoon to the Soviet Foreign Ministry, where an official, Vladimir Suslov, complained coldly about London's expulsion of the Soviets from Britain and handed Cartledge a list of British citizens who he said had been "engaging in acts incompatible with their status," which in diplomatic jargon could include spying.

The 18 British Embassy employes, five journalists and two businessmen are to leave Moscow within three weeks.

In London, the British government condemned today's expulsion by the Soviets as "totally unjustified" and indicated that it was considering new retaliatory measures of its own, Washington Post correspondent Michael Dobbs reported.

British Foreign Secretary Geoffrey Howe said in a statement that the Soviet action was bound to harm relations between London and Moscow. He said Britain was giving "urgent and careful consideration of the implications" of the Soviet expulsions.

George Younger, Britain's secretary of state for Scotland, immediately canceled a planned visit to Moscow this weekend and said he was doing so on the advice of the British Foreign Office.

Stunned that the Soviets reacted so swiftly and equally to London's expulsions, members of Moscow's British community interpreted the move as a show of strength on the part of the Kremlin that they said would probably further dampen Soviet-British relations, described by one western diplomat here as "lukewarm."

Cartledge told Suslov the actions were "wholly unjust" and denied the Soviet charges against the Britons.

Based on past experience and the fact that there are more Soviets stationed in London than Britons in Moscow, the British had expected a lower number of counter-expulsions by the Soviet Union.

When the British government expelled more than 100 Soviets in 1971, for example, 18 British subjects were asked to leave the Soviet Union.

Mark Frankland, the longtime Moscow correspondent of The Observer and one of those expelled today, attributed the directness of the reaction to the new Kremlin command under Soviet leader Mikhail Gorbachev. Frankland, who denied the espionage charge as "nonsense," said, "It's a new, vigorous leadership, which is given to responding to hard stances taken by the West with hard stances."

The expulsions deal a heavy blow to the British presence in the Soviet capital, which with 118 embassy staffers, journalists and businessmen already is one of the smallest in the close-knit western community.

The dismissal of 18 British diplomats and embassy staffers, and the expected departure of some of the diplomats' spouses, also employed by the embassy, will reduce the size of the British Embassy staff by more than one-fourth.

The British diplomats ordered expelled included three first secretaries -- Michael Asquith, Andrew Gibbs and Janet Gunn -- two second secretaries, six attaches and seven members of the administrative and technical staff. Asquith was one of the embassy's economic experts.

In addition to Frankland, the journalists expelled were Dennis Blewitt of the Daily Mail, Tim Sebastian of the British Broadcasting Corp. television, Alan Philps of Reuter and Robin Gedye of the Daily Telegraph, according to British Embassy spokesman Donald MacLaren.

The departure of their Moscow representatives could mean that the Daily Mail, Daily Telegraph and Observer will have to close their offices here, at least temporarily. Reuter still has four Moscow-based correspondents and the BBC has one.

The expulsion of Soviets from London on Thursday followed the defection to Britain of Oleg Gordievski, a newly apointed chief in Britain of the KGB, the Soviet secret police.

Without mentioning Gordievski, Tass, the official Soviet news agency, quoted the Soviet Foreign Ministry as saying that the expulsion of the Soviets by Britain was "a gross provocative action with regard to a number of Soviet representatives in Britain against whom ill-intentioned and absolutely unjustifiable accusations had been made."

British diplomats and journalists here originally expected a lighter response from Moscow, in part because London offered to allow Moscow to increase the size of its embassy, and in part because of a perception that Moscow is anxious to court western public opinion.

But Soviet official Suslov, in his half-hour meeting with the British ambassador today, described London's action as "hostile and malicious and designed to poison Anglo-Soviet relations," according to MacLaren.

After a period of some tension in relations in the early period of Conservative Prime Minister Margaret Thatcher's government in Britain, relations between Moscow and London improved following a heavily publicized visit to the British capital last December by Gorbachev, three months before he became general secretary of the Soviet Communist Party.

The British citizens who had been pinpointed on the list of expellees reacted with an awkward sadness to their imminent, unexpected departure from the Soviet capital.

"I am sorry, in many ways," said Blewett, a longtime correspondent here for the Daily Mail and other British newspapers, who, like others being expelled, denied the Soviet charges against him. Blewett added: "We've lost one of the cords of communication between East and West, and that's a pity."

Frankland, wrapping up the third year of his second assignment in Moscow said, "One is sad at the thought that one might never be allowed back."

Simon Priestly, head of Quest Automation Ltd.'s Moscow office and one of the two businessmen expelled, said, "I'm absolutely innocent. After 12 years here, the Soviets should know that perfectly well," The Associated Press reported.