Princess Beatrice put through her paces as Branson family get competitive

With just days to go until the London Marathon, Princess Beatrice has stepped her training up a gear as she prepares to take part in the annual race.

The 21-year-old royal has been seen out running with her fellow Team Caterpillar competitors ahead of Sunday's event, as she put in some last minute practice before attempting to run the 26 miles as part of a 30-strong human chain.

There is certainly a lot at stake.

As well as running to earn money for a number of charities, Beatrice and her team are going head to head with Virgin boss Richard Branson, whose company is sponsoring the marathon.

The British billionaire is heading up a group of celebrities, including Natalie Imbruglia and Scottish TV presenter Jenni Falconer, while his two children Sam and Holly will take their places alongside Beatrice in the caterpillar.

"We have a sportsman's bet that we are going to win against his team, although I am not sure we actually will," Sam told the Daily Mail.

And while they are technically pitted against each other, sometime model Sam is helping Australian beauty Natalie get ready for the big day. The pair have something of a shared history - they've been romantically linked in the past, and Sam has appeared in one of the star's music videos.

"Richard hadn't organised any joint training sessions and Natalie finds it difficult to run on her own – so she decided to join Sam's sessions in Hyde Park," a friend said of the pair, who have been practising together twice a week.