Posted on the BBC News website latest:
AT THE COURT
Emma Simpson Business correspondent, BBC News
The hearing is taking place in a packed small courtroom, tucked away in the east block of Royal Courts of Justice.
The big guns have been brought out to grapple with the legal arguments on this one.
The Master of the Rolls along with the Lord Chief Justice are on the panel.
So far much of this emergency session has focussed on the interpretation of section 231 of the 1992 trade union law.
It's technical, but there's a lot riding on the outcome and right now it's unclear how long this emergency application going to take.