LONDON, Sept 9 (Reuters) - The Brexit divorce deal contained ambiguities after being written at pace, Prime Minister Boris Johnson's spokesman said on Wednesday, adding it was always expected for the details on Northern Ireland to be hammered out after signing.

Johnson has been criticised for signing the withdrawal agreement, only to now introduce legislation that overrides parts of it.

"The Withdrawal Agreement and the Northern Ireland protocol, aren't like any other treaty. It was agreed at pace in the most challenging possible political circumstances, to deliver on a clear political decision by the British people, with the clear overriding purpose of protecting the special circumstances of Northern Ireland," the spokesman said.

"It contains ambiguities, and in key areas there is a lack of clarity. It was written on the assumption that subsequent agreements to clarify these aspects could be reached between us and the EU." (Reporting by Elizabeth Piper, editing by William James)