On Tuesday, Parliament's Environmental Audit Committee, which Mary chairs, published a report arguing that our EU membership is good for the UK's environment.
The Committee heard that EU environmental policy development has been a two-way street.
On the one hand, EU membership has given the UK a platform to pursue its environmental objectives internationally, and influence the strategic, long-term direction of policy.
On the other hand, EU membership has ensured that environmental action in the UK has been taken on a faster timetable, and more thoroughly than would otherwise have been the case.
Mary said: "The UK has cleaned up its act since we were dubbed the 'dirty man of Europe' in the seventies. EU environmental laws have played a key part, and mean we bathe on cleaner beaches, drive more fuel-efficient cars and can hold Government to account on air pollution."
Click here to find out more about the inquiry and to read the report.