Scotland Yard failed to tape-record its only interview with Leon Brittan concerning a rape allegation, according to a review that reveals yet more blunders in the controversial investigation.

Key questioning of the woman who accused the former Home Secretary of sexual assault was also of a ‘poor standard’, while files were left incomplete and important decisions made by junior figures, the report claims.

The errors will pile further pressure on the Met’s beleaguered chief, Sir Bernard Hogan-Howe, who is facing growing calls to quit over his force’s disastrous handling of historic sex abuse claims.

Scotland Yard failed to tape-record its only interview with Leon Brittan, left, concerning a rape allegation, according to a review that reveals yet more blunders and applies more pressure to the Met’s beleaguered chief, Sir Bernard Hogan-Howe, right

He is already due to apologise personally to Lady Brittan for not telling her husband before his death that he had been cleared of the rape allegation dating back to the 1960s.

The Met has also faced accusations that it only pursued the ‘witch-hunt’ against the Tory grandee because of pressure from campaigning Labour MP Tom Watson. But it can be revealed that a secret review of the case by a senior Dorset officer found several other ‘weaknesses’ including:

The first interview with the alleged victim was of a ‘poor standard’ while the second ‘lacked sufficient probing’;

The original officer in charge of the case admitted he was ‘inexperienced’ in rape investigations;

He made an ‘erroneous’ early decision that no offence could have taken place because of ‘his perceived issues with consent’, and there were ‘ample’ grounds to interview Brittan;

The eventual interview with Brittan by a second team of detectives was not tape-recorded because of ‘equipment failure’ – a ‘regrettable’ mistake that had a ‘significant impact’ on its ‘depth and quality’;

An independent assessment of the case should have been carried out to provide ‘rigour and integrity’ as it involved ‘very senior members of the British Establishment’.

However, the review, ordered by the Met and carried out by Dorset’s Deputy Chief Constable James Vaughan, claimed the investigation ‘broadly’ met police standards and was ‘launched in good faith against a credible account provided by a compelling witness’.

Some critics claim the review is a ‘whitewash’ that clears Sir Bernard and instead blames the original detective, DCI Paul Settle, who was sidelined.

Last night David Mellor, who served as a Minister under Brittan in the 1980s, said: ‘This is a cosy little set-up where a small county force is allowed to tell the Met they did a great job.