Why is there a Piccadilly Line Tube strike? Can I get a travel fare or Oyster ticket refund?

Piccadilly line drivers are staging a 48-hour strike causing severe disruption to the Tube service.

The walkout began at midday on Wednesday with services not expected to resume until Friday afternoon.

The strike will start again on Friday night at 8.30pm, affecting the Night Tube.

Stations particularly affected by the strike will include Finsbury Park, King's Cross St Pancras, Green Park and Hammersmith.

Members of the Rail, Maritime and Transport (RMT) union announced strike action after talks with TfL collapsed.

So why exactly are Tube staff striking and when will the Piccadilly line be operating as normal? Here's everything you need to know:

Why are Tube drivers striking?

Members of RMT have described a "complete breakdown of industrial relations" with TfL, blaming them for backtracking on operational, staffing and safety improvements.

The two sides met at the conciliation service Acas but talks concluded without an agreement.

Unions general secretary Mick Cash accused TfL bosses of "wrecking the planned Acas talks by denying reality and refusing point blank to make serious progress on the core issues that have reduced industrial relations on the Piccadilly line to a powder keg."

He added: "The company seem to think that they can promise improvements one day and then get away with playing fast and loose with safety and taking liberties with policies and procedures the next. Well, they can't."

Nigel Holness, TfL's director of network operations for London Underground, said TfL has made "good progress in making these changes."

Mr Holness also described the strikes as "unnecessary" and urged the union to "reconsider this action, withdraw the strikes and spare Londoners and visitors to the city needless disruption."

How do strikes work?

Industrial action happens when a dispute between union members and their employers can't be resolved through negotiations.

A trade union can only call for industrial action if a majority of its members offer their support via a properly organised ballot.

If you do take industrial action, you'll probably be in breach of your employment contract, which means your employer is unlikely to pay you for work you didn't do when you took industrial action and can sue you for breaking your contract (although this doesn't happen very often).

You can't be dismissed for industrial action if it's the result of a properly organised ballot, it's about a trade dispute between workers and their employer and a notice of at least 7 days has been given to the employer.

Can I get a travel fare or Oyster ticket refund?

Unfortunately, no. The TfL website states that they "do not give refunds for delays outside our control."

These include things like strike action, security alerts, bad weather, customer incidents and engineering works.

When does the Piccadilly line strike end?

A normal service is expected to resume between Heathrow and Cockfosters from 7.30am on Saturday 29 September.

By 8.30am, a normal service should be running between Acton Town and Uxbridge.

How can I keep up to date with tube strikes?

You can stay informed by following the TfL Twitter accounts @tfltravelalerts, @tfltrafficnews and @piccadillyline.