Lord Mandelson is due to address the conference on Monday
Business Secretary Lord Mandelson was initially refused entry to the Labour Party conference in Brighton because of a problem with his security pass.
A security guard called a police sergeant to verify his credentials, BBC political correspondent Laura Kuenssberg said.
Lord Mandelson had to wait 10 minutes at the conference's security entrance before he was allowed to enter.
The business secretary is due to give a speech to delegates on Monday.
'Up for grabs'
Being refused entry to Labour's conference could be seen as an inauspicious moment for one of the party's most powerful and prominent personalities.
Lord Mandelson has been widely regarded as a key figure in Gordon Brown's cabinet since the prime minister brought him back to government in 2008 after serving for nearly four years as an EU commissioner.
In June he was given increased powers as business secretary as well as the additional titles of First Secretary of State and Lord President of the Council.
Earlier, Lord Mandelson had insisted to the Sunday Mirror that the next general election was "up for grabs".
He told the paper: "This election is not in the bag - neither for us, nor the Tories.
"Every day we face in one way or another attempts to write Labour off or pretend that some of us are preparing for defeat.
"That's not my belief."