Mitt Romney, whose intolerance of illegal immigrants has been a central theme of his presidential campaign, was caught out for a second time employing undocumented Guatemalans to tend his lawn.

The revelation was an embarrassing setback for Mr Romney and a reflection of how the subject of illegal immigration has become one of the most perilous for Republican and Democrat candidates. The issue badly damaged John McCain's campaign and caused Hillary Clinton significant trouble last month.

The episode also threatens to overshadow Mr Romney's speech today in which he will formally address for the first time his Mormon faith.

Mr Romney announced that he had fired the landscaping company that provided the gardeners after The Boston Globe newspaper, for the second time in a year, revealed that illegal immigrants worked on his lawn and tennis court.

The first revelation was bad enough, with Rudy Giuliani making scornful reference to it in a debate last week. Mr Giuliani accused him of having a “sanctuary mansion” in response to Mr Romney's claim that he presided over a “sanctuary city” for illegal immigrants while New York's Mayor.

In a statement Mr Romney said: “After this same issue arose last year I gave the company a second chance with very specific conditions. They were instructed to make sure people working for the company were of legal status.” He described the recurrence as disappointing and inexcusable.

Mr Romney's stance on illegal immigration has been a core theme of his campaign and he has relentlessly attacked Mr Giuliani and Mr McCain over the issue.

Mr McCain is the only main Republican candidate who supported President Bush's efforts this year to give America's 14 million illegal immigrants a chance of citizenship. Republicans in Congress bowed to fierce opposition by conservatives and stopped the Bill but Mr McCain's support for it cost him dearly. His campaign nearly collapsed in the face of intense anger among Republicans, although it has rallied since.

Despite polls showing that most Americans support giving undocumented workers the chance to earn their way to legal status, nearly all the candidates seeking the Republican nomination have pandered to the party's base and decried what they call an “amnesty”. Immigration is a highly charged issue in Iowa, the first state in the contest. Winning Iowa is crucial to Mr Romney's campaign but he has been losing ground there in recent weeks.

Mr Romney's rivals have ridiculed him over the news. A spokesman for Fred Thompson said: “First Mitt Romney was for illegal immigrants working on his lawn, and then he was against it, then for it, and now I guess he's against it again.” An aide to Mr Giuliani said that the revelation spoke for itself. A laughing Mr McCain made lawnmower-pushing motions in front of reporters.

Hispanics are the largest, and fastest-growing, minority in America. Republican strategists believe that the anti-immigrant fervour poses a serious long-term threat if it pushes Hispanic voters into the Democratic Party. It is also an issue fraught with risk for Democrats. They do not want to alienate Hispanics, but many Democrat voters, particularly blue-collar and African-American voters, blame immigrants for driving down wages.

Mrs Clinton was heavily criticised in October for failing to say if she supported a plan to give driving licenses to illegal immigrants in New York.

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