West Ham United moved to the London Stadium from Upton Park in August 2016

London Mayor Sadiq Khan has ordered an investigation into a £50m increase in the cost of converting West Ham's London Stadium.

In 2015, former mayor Boris Johnson said the change from an Olympics arena to a football stadium would cost £272m, a figure which has now risen to £323m.

The investigation will look into inherited issues including the cost of moving the retractable seating.

A stadium expert has said the ground should be knocked down and rebuilt.

Paul Fletcher, who has built or advised on more than 30 new grounds, said it should be rebuilt as fans are too far from the pitch.

The club declined to comment when contacted by BBC Sport.

A spokesperson for the London Mayor said: "The mayor is deeply concerned about the finances of the Olympic Stadium, which have clearly been left in a total and utter mess by the previous administration at City Hall.

"Sadiq has ordered a detailed investigation into the full range of financial issues surrounding the stadium."

Part of the reason for the increase is a rise in the cost to install and operate the retractable seating, which can be removed for concerts and athletics events.

The cost of £8m a year is up from an estimated cost of £300,000 because the company originally contracted for the job had gone bust, reports BBC sports editor Dan Roan.

It means stadium operators London Legacy Development Corporation (LLDC) and the London taxpayer will need to pick up the additional costs, with Premier League side West Ham contributing £2.5m-a-year rent, plus the one-off £15m for the conversion cost.

Sir Robin Wales, Mayor of Newham - the borough which co-owns the stadium after investing £40m - said he "fully supported" the call for an independent review into the rising costs.

"We have a duty to taxpayers to scrutinise this," he added.

John O'Connell, chief executive of the TaxPayers' Alliance, said: "For too long the details of this shabby deal were kept in secret and lacked proper scrutiny so Sadiq Khan is absolutely right to look again at the case.

"It's now down to those in charge of the inquiry to get to the bottom of how West Ham were gifted the deal of the century while ensuring their investigation doesn't end up costing the taxpayer as much as the stadium."

West Ham won the bid to occupy the Olympic Stadium in March 2013 and played their first game there in August 2016, after 112 years at Upton Park.

However, their move has been marred by fan violence, leading the club to issue a five-point security plan to prevent further disorder.

Analysis

Dan Roan, BBC sports editor

It may have been the centrepiece of London's glorious 2012 Games, but the story of the Olympic Stadium is becoming more and more troubled.

Last year, the BBC revealed West Ham were getting the running costs of their new home paid for them. Now its suitability as a football venue is under fierce scrutiny because of disorder at matches. A proposed naming rights partner has walked away, while there is renewed focus on the finances of a stadium that has had to be built twice, at an overall cost of £750m.

The soaring costs effectively end any hope of the stadium breaking even within five years. The LLDC will try to reduce running costs and look to install more efficient hydraulic retractable seating. But the fear will be that, until then, the staging of some summer concerts and events - crucial for the stadium's finances - will be affected because of the time it takes to reconfigure the arena at the end of each season.

Once again, there will be tough questions about those originally responsible for the stadium's legacy, the remarkable deal West Ham struck, and why it was not designed for dual-use from the very beginning.