LONDON, Feb 14 (Reuters) - Government delays, uncertainty and under-resourcing have left the British borders and immigration system unprepared for Brexit, according to a parliamentary report on Wednesday.

Lack of clarity over immigration intentions is creating anxiety for EU citizens in the UK and putting already overstretched immigration officials in an impossible position, it added.

"The government does not seem to appreciate the immense bureaucratic challenge they are facing or how much time and resources they need to plan on Brexit," it said.

The report by the House of Commons Home Affairs Committee criticised delays in publishing a so-called White Paper outlining the government's post-Brexit immigration plans.

Prime Minister Theresa May's office has said it is considering various options for the post-Brexit immigration system and will set out its initial plans "as and when they are ready."

But the committee said failure to set out immigration objectives soon will deny parliament and those affected the chance to debate plans before they are finalised.

"That is unacceptable," it added. "We had expected these questions to be answered in the much-delayed White Paper but its publication has been delayed further and it may not now appear until the end of this year at the earliest."

May has warned that European citizens arriving in Britain after Brexit next year may lose some rights, setting up a clash with the EU over their treatment during any transition period before leaving the bloc.

Curbing immigration was a key reason why Britons voted to leave the EU in 2016, following a large influx of EU citizens, especially from poorer countries in eastern Europe.

"With little more than a year to go, the government is still failing to set out crucial details on the registration of current (EU) residents," the report said.

Failure to set out detailed plans for the registration of foreign nationals and the transition period soon will make it impossible for immigration and border officials to do their job properly, it warned.

The government needed to clarify urgently whether it wants additional border checks after March 2019 when Britain leaves.

It should aim to agree transitional arrangements with the EU which involve no practical change to customs operations, it added. "Removing border staff from security or immigration checks to do additional customs checks would be unacceptable."

"Government drift is putting everyone in an impossible position," said committee chair Yvette Cooper. "The lack of detail with just over a year to go is irresponsible." (Reporting by Stephen Addison; Editing by Andrew MacAskill)