Mr Major refused to condemn Labour for promising to abolish the "primary purpose" rule in the immigration laws, as reported in Monday in The Independent.
The rule requires immigrants to prove that a marriage was not for the primary purpose of gaining a right to live in the UK. It is attacked in an official guide to Tory candidates for the forthcoming election campaign.
But Mr Major refused to go onto the offensive when Nicholas Budgen, the Tory MP for Wolverhampton South West, urged him to condemn Labour for threatening to liberalise the immigration controls which Mr Budgen said had helped to reduce racial tension in Britain.
There were cries of "you're desperate" by Labour backbench MPs, and shouts of "racist" at Mr Budgen's question. But Mr Major replied: "In the last 18 years, we have seen the most extraordinary changes and improvements in race relations in this country. That is immensely important. I certainly am not going to lend my voice or my policy to anything that would change that improvement."
Tony Blair took the unusual step of praise Mr Major for his reply. "I think he deserves credit for that answer," said the Labour leader.
However, it dismayed other rightwing Tory MPs. "The Prime Minister is trying to distance himself from it. He doesn't want to upset the immigrant community in the run-up to the general election. That is what he was doing in putting Nick Budgen down and then Blair gave him a patronising pat on the back," said one of Mr Budgen's friends.
The Conservative Party campaign guide to be used by Tory candidates, warns in a chapter on race relations that Labour's plan to abolish the primary purpose rule and grant immigration rights to extended families "would undoubtedly lead to increased immigration and fresh abuse of the system."
Mr Major's refusal to use that formula suggests that the Prime Minister, who was brought up in Brixton, personally wants to avoid race becoming an election issue, although it was proposed by a former Tory Central Office research chief.
A senior Tory source said: "The Prime Minister comes from a multi-cultural background. He believes in firm but fair immigration rules. However, he does believe that reducing them, as Labour propose, would endanger good race relations."
Both Labour and the Tories have been energetic in recent months in seeking the support of the ethnic communities in Britain, particularly the Asian vote, which could prove decisive in marginal seats in the Midlands.
An Asian lobby group, the Confederation of Indian Organisations, is preparing to target ethnic voters in Britain with pamphlets carrying the Tory response to Labour's proposals for liberalising the immigration rules. It will be used in eight marginal seats where the swing of a few hundred votes could oust Tory MPs.
While Mr Major was reluctant to go onto the attack on immigration, Michael Howard, the Home Secretary, in response to the Independent report, issued his own condemnation of Labour's plans for scrapping the asylum controls.
He said scrapping the 1996 Asylum Act would cost the taxpayer an extra pounds 200 million a year in extra benefits. Labour's policy to drop the requirement on employers to make checks on illegal immigration would be "a charter for the sweat shop and it would threaten the jobs and living standards of British citizens."
n A Tory rightwinger was under attack from both sides last night after describing the Prime Minister as "vindictive and unforgiving" and the Heritage Secretary as "dead from the neck up."
David Evans, the MP for Welwyn and Hatfield, also referred in an interview to the "three bastard children" of his Labour opponent at the general election and talked of "some black bastard" who had raped a girl.Reuse content