The home secretary is facing pressure to explain her handling of the decision to relax some border checks after the UK border force head stepped down.
Brodie Clark has contradicted Theresa May's version of events, accusing her of misleading Parliament.
But, as home affairs correspondent Danny Shaw explained, there may be a "grey area" in the running of the border agency which led to the disagreement.
Shadow home secretary Yvette Cooper said that the truth was that "we don't know how many people these reduced checks applied to".
She argued that over the summer, the home secretary went against years of political agreement to continuously strengthen the UK's borders and then "did not keep track of it".
"We know she has undermined the checks, but she hasn't revealed the full truth," she said.
"We've got an escalating fiasco in which we have a contradictory story between her and her most senior official.
"If she doesn't tell the truth today and publish all of the data... then we're going to see a continual sapping of the home secretary's authority."
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