Although he has been persuaded to return to the show for for a third time, Sir Alan disclosed yesterday, his eventual agreement came only after being consulted about the calibre of the potential candidates.

More than 10,000 people applied to take part in the new series, which has transferred from BBC2 to BBC1, where candidates battle for a £100,000-a-year job working with Sir Alan. He gave advice as to what questions should be asked to whittle the masses down to 16 hopefuls and avoid a repetition of his "disappointment" with the standard of candidates on the second series.

The shortlisted candidates, who include a bankrupt entrepreneur who employed public school-educated staff, including Kate Middleton, to serve drinks at Henley, a quantum physicist and three single mothers, shared a house in Notting Hill while taking part in the "job interview from hell".

The candidates are set tasks that include selling British produce at a French farmers' market, designing accessories for dogs and selling sweets at London Zoo. Sir Alan told them: "This is no game show - there won't be some busty blonde waiting outside to hug you so that you can sob into her bosoms. What there will be is a black cab waiting to take you to the station."

He is convinced that the contestants from the third series are of a "very high" standard. "As time goes by, you can smell someone who is there for the right reason," he said. "Part and parcel of me agreeing to do a third series was that the calibre needed to be looked at very carefully."

Sir Alan praised Tim Campbell, the winner of the first series who still works for Amstrad, who he described as "a wonderful fellow, a role model for young people and a great chap". But when asked about the second series, he said: "I always have the problem of wondering if [candidates] are here for the right reasons. As I've seen some of them try to capitalise on the fact they have been on TV, it's something I think I've got to learn about in making sure the selection this time is people who are there for the right reason.

"The power of TV is massive. You take these people out of a normal job, they're in the press, they see themselves splashed on the screen, they see all the attention that's paid to them and they go in the wrong direction."

The former supermarket checkout worker Michelle Dewbury, who he chose to become his apprentice in the second series, quit her £100,000 job with Sir Alan and has since written her autobiography and appeared on a BBC celebrity hairdressing show. She also had a well-publicised affair with one of the other candidates, Syed Ahmed.

Asked if he regretted not choosing Ruth Badger, the runner-up, as his apprentice, Sir Alan replied: "There's a lot of regrets about last year. Maybe turning up was the biggest regret about it. I'm in an awkward situation in having to bite my tongue - that's the best thing I can say at this moment in time."

But he believes The Apprentice, along with Dragon's Den on BBC2, "inspire a lot of people". "I take my hat off to the BBC with programmes like Masterchef [and Dragon's Den]. There's an air of quality, a lack of this trash reality show about them," he said.

The third series of The Apprentice starts on 28 March on BBC1

Hired or fired?

Tre Azam, 27, from Loughton, Essex, a marketing and design consultant.

Katie Hopkins, 31, from Exeter, a single mother-of-two and global brand consultant.

Kristina Grimes, 36, from Harrogate, a pharmaceutical sales manager and a "ruthless single mother".

Rory Laing, 27, from Bristol, a waiter and bankrupt entrepreneur who employed ex-public school pupils including Kate Middleton for the Henley regatta.

Lohit Kalburgi, 25, from London, a telecoms manager born in the United Arab Emirates.

Adam Hosker, 27, from Lancashire, a car sales manager.Lives with wife and children in Blackburn.

Natalie Wood, 29, from Upminster, Essex, represented England in swimming as a teenager. The mother-of-two has worked in the City.

Paul Callaghan, 27, from Southampton, an ex-Army lieutenant who graduated from Sandhurst and spent six months serving in Basra.

Jadine Johnson, 27, from Harrow, Middlesex, a financial adviser and single mother.

Gerri Blackwood, 33, from Woking, Surrey, a transport development manager. Turned down a job at MI5 for The Apprentice.

Dr Sophie Kain, 32, from Llanellen, Wales, a research scientist for aviation firm "who doesn't suffer fools gladly".

Ifti Chaudri, 33, from Egham, Surrey, a company director of tile business. Applying to join the 2012 Olympic team.

Andy Jackson, 36, from Leeds, a car sales manager now living in Kirriemuir in Scotland.

Ghazal Asif, 23, from Glasgow, a business development manager. Speaks five languages.

Naomi Lay, 26, from Cornwall, an advertising sales manager. Has run both the New York and London marathons.

Simon Ambrose, 27, from Clapham, south London, a former investment banker, runs internet-based businesses.