Fiona Bruce had to flag down a police car so she could get to another studio to present the News At Six today.
The BBC team decided to head to its Millbank unit after a technical glitch caused problems at the BBC Broadcasting House.
But the newsreader found traffic at a standstill so persuaded a police officer to get her to the other studio with minutes to spare.
The journalist, who also hosts Antiques Roadshow, said: Ive never had to do that before in 19 years of presenting the news.
Mark Easton, BBC News home editor, thanked policeman Josh in a series of tweets after the frantic journey in the capital.
He wrote: In a taxi with Fiona Bruce and editor of BBC News At Six heading for Millbank. Studio at Broadcasting House gone down!! Traffic dreadful. Aagghhh.
Now in a police car being escorted through the demo. BBC News is at stake!
And later he added: Policeman Josh has saved the day! BBC News should be on air!!
The BBC had to run previously recorded content on its live news channel due to the technical issue.
A BBC spokesman previously said: Were working hard to resolve this as quickly as possible. The BBC News website is operating normally.
The glitch appeared to last for around one hour before the news channel returned to air.