Queen Elizabeth attends opening of London Tube line

Britain's Queen Elizabeth on Tuesday made a surprise appearance at a ceremony to mark the completion of a long-awaited London Tube line named in her honour.

Elizabeth, wearing a bright yellow hat and coat and using a walking stick, unveiled a plaque at Paddington Station. She was joined by her son Edward and Prime Minister Boris Johnson.

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It is the latest public outing for the 96-year-old monarch who, despite missing a speech in parliament last week due to mobility issues, twice attended a horse show at her Windsor Castle residence in recent days.

"In a happy development Her Majesty The Queen is attending today's event to mark the completion of the Elizabeth line," a Buckingham Palace spokesperson said.

The $24 billion rail project was originally due to be opened in December 2018, but had been repeatedly delayed by issues with safety testing and signalling systems, even before the onset of the pandemic.

Originally named 'Crossrail', the Elizabeth line will connect destinations west of London, including Heathrow airport and Reading with Shenfield in the east. It will open to the public on May 24.

Elizabeth has been seen in public on only a few occasions after she spent a night in hospital last October for an unspecified illness, after which she was told to rest by her doctors.

She has since carried out duties either remotely or in person at her Windsor Castle home and did not attend last week's state opening of parliament.