The Daily Mirror reporter said he quit the royal staff the night before he was to bring breakfast to Secretary of State Colin Powell and National Security Adviser Condoleezza Rice on their first morning at the palace.
The Mirror splashed the headline "Intruder" in big, red letters across its front page Wednesday with a photo of its reporter, Ryan Parry, standing on a Buckingham Palace balcony.
Bush — who arrived Tuesday in London for a state visit and is staying at the palace as a guest of the queen — was informed of the breach Wednesday morning.
"We have every confidence in the British security," White House spokeswoman Claire Buchan said, referring all other questions to British officials.
Buckingham Palace was conducting a "full investigation," a royal spokeswoman said. Home Secretary David Blunkett, whose department is responsible for law and order, said, "We are satisfied that both the security and the criminal records checks were done robustly and correctly and that there was no risk from this individual."
He emphasized that the Daily Mirror reporter had lied only about his work history. Blunkett said officials had resolved problems with physical security at the royal palaces, but conceded the failure to fully check the reporter's background "is a breach and ... it needs to be closed."
He added that the independent Security Commission would also review the apparent gap.
Prime Minister Tony Blair, asked about the report in the House of Commons, said, "I think it's important that we establish the facts first."
"Right royal fiasco," a page-two headline in the Mirror said, starting 15 pages of coverage, featuring teasers like "Inside the president's bedroom" and "I could have poisoned the queen."
The Mirror said Parry worked as a royal footman at the palace for two months.
"We keep being told this is the biggest security operation of all time and it seems to me to beggar belief that we could get to a situation, as we did last night, where our journalist was detailed with serving breakfast on the presidential floor this morning and at the state banquet tonight," Mirror editor Piers Morgan said.
"Worse than that, he was given the charge last night, when the presidential party arrived, of going in and leaving their chocolates and nuts, completely on his own. He could have done anything," Morgan said.
The Mirror published photos said to depict the rooms in Buckingham Palace where Bush and his wife, Laura, are staying, and said Parry had a full view of the presidential couple arriving Tuesday.
"Had I been a terrorist intent on assassinating the queen or American President George Bush, I could have done so with absolute ease," Parry said.
"Indeed this morning I would have been serving breakfast to key members of his government, including National Security Adviser Condoleezza Rice and U.S. Secretary of State Colin Powell," he added.
David Davis, the opposition Conservative Party's spokesman on law enforcement, said the reported infiltration exposed a "potentially fatal weakness at the heart of security covering the head of state."
Tory lawmaker Patrick Cormack asked Blunkett at the House of Commons if he was satisfied that everyone working in the palace, including those serving at the state banquet Wednesday, had been properly vetted.
"Yes, I am," Blunkett responded.
In June, a comedian dressed as Osama bin Laden gatecrashed Prince William's 21st birthday party at Windsor Castle in what Blunkett called "an appalling failure" in royal security.
Security at Buckingham Palace was tightened in 1982 after a mentally disturbed man, Michael Fagan, climbed a drainpipe and spent 10 minutes sitting on the queen's bed, holding a broken ashtray and talking with her before guards arrived.
The worst breach of royal security occurred in 1974 when gunman Ian Ball tried to abduct the queen's daughter, Princess Anne, as she and her first husband, Mark Phillips, were being driven to the palace. Ball forced the limousine to stop and brandished a pistol. Anne and Phillips were not harmed, but her bodyguard was shot and wounded. Ball was sent to a mental hospital.
In 1993, anti-nuclear demonstrators scaled the palace walls with ladders and sang on the lawn before being arrested. In 1994, American paraglider James Miller landed on the palace roof as a stunt. In 1995, a man rammed the palace gates in a car. An escaped mental patient got into the grounds of Buckingham Palace in 1997 but the queen was away.