He admits it is ‘the biggest gamble of his life’ and is terrified that his latest television venture will be a flop.

And it would seem comedian James Corden, who makes his debut this month as anchor of The Late, Late Show – one of America’s top talk shows – may be right to be concerned, according to some members of the test audience at a rehearsal last week.

Corden and his new employers, CBS, could be worried that American audiences won’t get his British brand of humour and they want to ensure he is word perfect for his first live show on March 23, when Hollywood superstar Tom Hanks will be a guest.

Response: James Corden is to make his debut as anchor of The Late, Late Show later this month - but CBS bosses could be worried about how he will be received, so a rehearsal was held this week

A source on the programme revealed they needed to ‘iron out the glitches and find out what works’, adding: ‘The US audience is totally different to anything James has experienced before. It’s been a real rollercoaster of highs and lows.’

The Mail on Sunday gained exclusive access to Corden’s rehearsal last week, and while some of the 200 invited audience – many of them professional actors paid to ‘fill’ seats – seemed charmed by his laid-back delivery, most complained that they were ‘baffled’ by his sense of humour and found his estuary accent difficult to understand.

Though he told interviewers last week that he intends to ‘shake up’ late-night TV in the US, the new show is surprisingly formulaic.

Corden, 36, sits behind a desk topped with an old-fashioned rotary phone, while guests sit on a blue sofa in front of a fake backdrop of Los Angeles. There is a small bar set up in the corner of the studio, staffed by barman Ian, and a house band led by wild-haired musician Reggie Watts.

The comic opened the ‘show’ with a monologue of jokes, including one about last week’s scandal over US Secret Service members who crashed into a White House barrier after a drunken party. ‘They’ve got a new motto – to protect and serve me another,’ he told his audience.

Guest Susan Wittan, from Rockville, Maryland, who attended Thursday’s rehearsal said: ‘I’d never heard of James Corden until today.

'He seems to be a very friendly, nice guy but I didn’t get some of his jokes and I found him a bit hard to understand at times. He speaks quite quickly and used British words like mate.

‘I enjoyed some of the humour but I’m not sure how well he will go down in America.’

Revolutionary: Despite Corden - seen here interviewing Whitney Cummings, of sitcom 2 Broke Girls, and Eric Christian Olsen, from crime show NCIS: Los Angeles - promising the shake up the tried-and-tested format, some audience members complained there was little to differentiate it from other talk shows

Another woman, Christine, who revealed she was being paid as a ‘seat filler’ said: ‘I don’t think enough people know who he is. I didn’t enjoy the show much. It was boring.’

The dress rehearsal came amid increasing concerns that Corden, who has lost 6st in the run-up to the show’s launch, has been under tremendous stress, doubtless mindful that many other Britons, such as Ant and Dec, and Russell Brand, tried and failed to crack the American audience.

While friends and family insist his new, slimline image is the result of a conscious effort to look svelte for his big launch, others close to the show say his fear of failure is the real reason that he has lost so much weight.

Risk: Corden has moved his family - wife Julia Carey, son Max, three, and four-month-old daughter Carey - to LA, but admits taking the role is the biggest gamble of his life so far

In recent weeks he has displayed obvious signs of anxiety.

‘There have been a few heated moments,’ the source said. ‘There have been a few raised voices. But that can be normal for any new show.

‘You can’t blame him for being stressed and nervous. His entire professional future is on the line.’

Corden, who relocated to LA at Christmas with wife Julia and their children Max, three, and 14-week-old Carey, is on a reported £1.5 million-a-year deal.

He admits CBS has taken ‘a huge gamble’ bringing him to the United States, saying: ‘I don’t know if I’m making a huge mistake. But I’d rather regret doing something than not doing something.

‘It’s madness really. When I got the job I’d never even been on an American talk show.’

Before Thursday night’s rehearsal began Corden, clearly on edge, spoke to the audience.

He told them: ‘This is a dream come true for me. If you’d told the 12-year-old me that I would be doing this I would never have believed it.’

He also spoke about his children, saying: ‘When you have one kid it’s like having a pet. When you have two it’s like getting the whole zoo.’

But he appeared distracted at times, with one onlooker commenting: ‘He seemed like he had a lot on his mind. There were times during the moments by himself when he looked stressed and worried.’

He interviewed comedian Whitney Cummings, of sitcom 2 Broke Girls, and actor Eric Christian Olsen, from crime show NCIS: Los Angeles. Both are CBS shows.

When Cummings joked about Britain being ‘a nation of alcoholics’ Corden replied: ‘I would refute that although I am aware that I’m the only late-night host with a bar in his studio.’

Family man: Corden told the audience about his newly expanded brood, saying: ‘When you have one kid it’s like having a pet. When you have two it’s like getting the whole zoo.’

Unknown?: Some said they thought Corden was not well-known enough to do well as a talk show host in the States - but CBS have been doing a lot of promotion to spread his name around, like this billboard stunt

Afterwards one British audience member said: ‘The show was pretty predictable. The main difference between this one and the other American late-night shows is that the guests walked out and sat on the sofa together. It’s more like Graham Norton’s show in format.

‘There were skits and jokes and a house band. He made the effort to engage the audience and invited everyone up on to the stage at the end to dance.

'But there’s nothing revolutionary about it. He’s likeable but won’t win any originality awards. He was trying very hard, though. The tone of the show is sweet.’

Amelia Darwin, who also watched the show, added: ‘James Corden worked really hard and is a likeable man. But I’m not sure he’s got what it takes to be a huge success here. The show is too similar to everything else out there.

'I wish him well but I won’t be tuning in.’

Nick Bernstein, CBS Vice President, Late Night, West Coast said: 'The studio audience reception to the practice shows has been terrific. 80% of the audience has re-booked for a future taping.

A late night host with an accent is hardly a foreign concept to CBS. We find James British accent quite charming.'