Sir Ian Blair, the Metropolitan Police commissioner, has ordered an inquiry after an undercover reporter posing as a delivery driver drove a hired van, carrying a brown box labelled "Bomb" into the grounds of Windsor Castle just days before the royal wedding.

Alex Peake, a reporter for The Sun, said that it was "absurdly easy" to breach the castle's security and drive past the chapel where the marriage of the Prince of Wales and Camilla Parker Bowles will be blessed after their wedding on Saturday.

A Scotland Yard spokesman said: "This apparent breach of security at Windsor Castle in the run-up to the royal wedding properly raises serious concern. It is only right that the facts are established before any action is taken against any person who may be culpable."

The newspaper described how Mr Peake and photographer Gary Stone dressed in workmen's overalls and used a counterfeit delivery note to hoodwink Royalty and Protection Squad officers guarding the King Henry III gate.

They were given directions to the castle's Saxon Tower as they were waved through in their spray-painted Thrifty van. They drove past the chapel and the Round Tower - where the Royal Standard was flying suggesting that the Queen was in residence - before stopping to take pictures, which were later shown to police.

The newspaper stunt follows a breach earlier this week when two tourists were caught wandering in a restricted area despite a reported £5 million security operation in the run-up to the ceremony.

It will add further to the embarrassment over the couple's wedding plans, which were hastily rearranged to prevent a clash with the funeral of Pope John Paul II tomorrow.

Mr Peake reported: "It was absurdly easy. Had it [the fake bomb] been real, it could have devastated the castle and caused carnage - even killing the Queen."

The self-styled "comedy terrorist" Aaron Barschak was involved in another high-profile security breach when he gatecrashed Prince William’s 21st birthday party in 2003.

Mr Blair, who warned in February that the wedding was a potential terrorist target, has ordered an "immediate inquiry to establish these facts".

A spokeswoman for Buckingham Palace said: "Security is a matter for the police who have been asked to investigate."

Weather forecasters have added to the couple's misery by predicting bitter winds and possibly snow as arctic weather sends temperatures plummeting to 6C on Saturday.

Meanwhile, an ICM poll has predicted that 18 million people will tune in for the Grand National, twice the number expected to watch the blessing, which will be broadcast live from the chapel.

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