BRITAIN'S ruling Conservatives think they may have found the issue most likely to give them a decisive edge in the coming general election: the low public approval ratings of Neil Kinnock, the Labour Party leader, compared with Prime Minister John Major's ratings.Party officials are saying the Labour leader's low ratings offer Mr. Major the opportunity of pitting himself successfully against Mr. Kinnock in a US presidential-style contest in which personal images rather than party policies could be the determining factor. What they decline to say is that it may prove easier to project the prime minister's favorable image than to argue that Conservative policies hold the key to an early British economic recovery. A public opinion poll last weekend showed that only 23 percent of those questioned thought Kinnock, the longest-serving opposition leader this century, would make a good prime minister. In another poll 38 percent said Kinnock was the main reason why they did not intend to vote for Labour in the general election which must be held before mid-1992 but may be called earlier. The equivalent figure for Major was 8 percent. The same poll showed that 25 percent of those questioned would be prepared to switch to Labour if Kinnock were replaced. Conservative party officials say the poll findings, coming as the annual season of party conferences gets under way, offer a solid basis for Major to run against Kinnock in a campaign designed to exploit the Labour leader's relative unpopularity with voters - a strategy preferred by party chairman Chris Patten. On Tuesday Kinnock summoned his shadow cabinet to a strategy session amid persistent rumors that Major was under pressure to call a snap general election for Nov. 7. Before the meeting the Labour leader dismissed as "absurd" suggestions that he was being asked by colleagues to step down and let someone with a better public image take over. "I have been, and am, a very good captain of the team. You do not drop winning captains - and we will win," Kinnock said in a speech in Birmingham. Labour officials claimed Kinnock was the victim of a Conservative attempt to divert attention from the recession. John Cunningham, the party's campaign director, said: "It is because he has brought Labour to the brink of a general election victory that much of this abuse and unfair treatment continues." Kinnock's image problem is explained partly by the long period he has been Labour's leader. He took over in October 1983 after his party had suffered its worst election defeat since World War II. Chosen to challenge former Prime Minister Margaret Thatcher, he led Labour to another spectacular election defeat in 1987, when the Conservatives won a majority over Labour of more than 100 seats. Since then he has concentrated on moving Labour toward the political center by abandoning nationalization and unilateral nuclear disarmament, loosening its ties to trade unions and pushing left-wing extremists out of the party. But many voters perceive him as a leader who has been around a long time, whereas Major, who succeeded Mrs. Thatcher last December, has a newer image which he has been burnishing by a spate of high-level international travel since Parliament recessed last July. Roy Hattersley, Kinnock's deputy, said Major's popularity could be explained by the "halo effect" of being in the public eye during the Kremlin crisis and its aftermath. Other observers think issues rather than personalities will be the chief factor influencing voters once the campaign gets under way. David Cowling, a poll analyst, says that after the 1987 general election only 8 percent of voters gave the party leaders' personalities as the decisive reason for voting as they did. In the 1970 general election the Conservatives, under the unpopular Edward Heath, defeated Labour. In 1979 Thatcher was much less popular than James Callaghan, the Labour prime minister, but won the general election against a background of industrial unrest and economic difficulty. Kinnock appears to believe that he can upset the pollsters by concentrating on the Conservatives' record of high unemployment and highlighting the persistent economic recession. "The leadership that most concerns the British people," Kinnock told reporters, "is the one that has brought a 700,000 rise in unemployment, a 50,000 rise in company bankruptcies, a 70,000 rise in household mortgage repossessions, a 20 percent fall in investment, and put a million people on hospital waiting lists. That's the leadership of John Major and his government." But inflation is continuing to fall, and now stands at 4.5 percent - the lowest for three years. With the economic indicators mixed, the prime minister is being urged by Mr. Patten and other senior Conservative officials to give full consideration to his personal popularity as a possibly decisive factor in the coming campaign. But asked about a possible election date, Major replied: "I'm in no hurry."