Image copyright Getty Images Image caption Chris Christie said it was time for the justice system to "do its job"

A former aide to New Jersey Governor Chris Christie has pleaded guilty to conspiracy charges over a traffic jam on the George Washington Bridge, a major access route to New York City.

David Wildstein admitted he played a part in shutting down one of the busiest crossings in the United States in September 2013.

The gridlock was allegedly engineered to punish a Democratic mayor who did not endorse the governor's re-election.

Mr Christie has denied any involvement.

Bill Baroni, former deputy executive director of the Port Authority of New Jersey and Bridget Kelly, a former deputy chief of staff to Mr Christie, were also charged with conspiracy to commit wire fraud.

According to the charging document, Mr Wildstein, Ms Kelly and Mr Baroni conspired to punish the mayor of the New Jersey suburb Fort Lee for refusing to support Mr Christie's re-election bid.

They are alleged to have shut down two lanes of traffic on the bridge on the first day of school to maximise the congestion. Commuters were stuck in traffic for hours over four mornings.

Both Mr Baroni and Ms Kelly have denied the charges.

Image copyright AP Image caption Alan Zegas (R) said his client David Wildstein (L) regretted playing a part in the bridge closure

Mr Wildstein, a former senior official at the agency that operates the bridge, pleaded guilty to two counts of conspiracy in a court in Newark on Friday.

He could be jailed for up to 15 years, although his guilty plea may be taken into account in sentencing. His lawyer Alan Zegas said his client "deeply regretted" his involvement in the incident.

Mr Zegas also claimed that "evidence exists" to prove that Mr Christie was aware of the lane closures while they were occurring.

However, the governor again rejected the allegations.

"Today's charges make clear that what I've said from day one is true," he said. "I had no knowledge or involvement in the planning or execution of this act."

The US prosecutor for New Jersey, Paul Fishman, said there were no plans to charge anyone else in connection with the scheme.

Mr Christie was once a favourite for the Republican presidential nomination, but has seen his popularity dip in recent months.