Downing Street was last night facing the threat of a police investigation over claims that the Prime Minister's chief of staff hid more than £700,000 of donations to boost Keir Starmer's political career.

The Tories say they have obtained private legal advice to Morgan McSweeney, Sir Keir's most powerful aide, which points to a deliberate attempt to mislead the Electoral Commission over money given to Labour Together – the think-tank which played a key role in bringing down Jeremy Corbyn and putting Starmer into power.

The Electoral Commission found more than 20 breaches by the group, and levied a fine of £14,250 in September 2021, after explicitly telling Mr McSweeney in 2017 it must declare donations within a 30-day limit.

However, dozens of donations made to Labour Together between 2018 and July 2020 were not declared until after Mr McSweeney left the organisation that year.

Now the Conservatives say private legal advice given to Mr McSweeney contradicts Labour Together's public claim the mistakes were due to 'human error and administrative oversight' and it had been as 'open and transparent' as possible.