LONDON (REUTERS) - British Prime Minister Theresa May is under pressure to introduce legislation guaranteeing promises made to long-term residents from the Caribbean who have been incorrectly labelled illegal immigrants.

For almost two weeks, British ministers have been struggling to explain why some descendants of the so-called "Windrush generation", invited to Britain to plug labour shortfalls between 1948 and 1971, are being denied basic rights because of trouble documenting their status.

The Windrush scandal overshadowed the Commonwealth summit in London earlier this month and has raised questions about Mrs May's six-year stint as Interior Minister before she became Prime Minister in the wake of the 2016 Brexit referendum.

The government has apologised for the fiasco, promised citizenship and compensation to those affected, including to people who have lost their jobs, been threatened with deportation and denied benefits because of the errors.

But more than 200 mainly opposition members of Parliament have written to the Prime Minister calling for the pledges to be written into law "without delay".

The letter accuses the Interior Minister, Ms Amber Rudd, who has faced calls to resign over her handling of the scandal, of making up policy "on the hoof" to defuse the situation.

Ms Rudd is due to address Parliament on Monday (April 30) after giving contradictory statements about whether the government has immigration removal targets.

In a separate letter, the shadow interior minister, Ms Diane Abbott, has called for a full inquiry into whether the Interior Minister misled Parliament and breached the ministerial code.

Ms Abbott said Ms Rudd's explanations as to why she had told members of Parliament there were no such targets, when in fact there were, "stretch credulity to the limit."

But Mrs May's office said there is no need for an investigation because ministerial code only requires a resignation if a minister knowingly misleads Parliament - a charge the Interior Minister denies.