The Met Police are being faced with legal action over its decision not to launch an investigation into the 'cash for honours' row.
Critics called for police to investigate claims that the Tory party has systematically handed out peerages to major donors who gave more than £3m to the party.
SNP Commons leader Pete Wishart wrote to the Met Police, calling for it to launch an investigation into Boris Johnson as well as previous Tory prime ministers and Conservative party officials but the force later said it did not have sufficient grounds to launch a probe.
Wishart and the Good Law Project, which has brought several civil cases against the Conservative government, have now written to the Met Police threatening judicial review if they do not explain why they refused to investigate.
A post on the Good Law Project website said: "If the Metropolitan Police refuse to investigate, they must satisfactorily explain why or risk judicial review.
"Alongside Pete Wishart MP, we are asking the Metropolitan Police to share the information and documents they considered before refusing to investigate, and the internal record of the refusal."
Last week Boris Johnson defended the practice of major Conservative donors being given seats in the House of Lords, saying as long as trade unions funded other parties, the system would have to continue.
In an exchange while giving evidence to the Commons Liaison Committee — the group of select committee chairs — on Wednesday, the prime minister said those who had been given peers had done "acts of great public service".
Watch: Met Police urged to launch 'cash for honours' investigation
Sharing the news about its threat of legal action, Good Law Project director Jo Maugham tweeted: "Boris Johnson has more or less admitted selling seats in the House of Lords for donations to the Tories. That's a criminal offence but @metpoliceuk have refused to investigate. With @PeteWishart, we've written to the Met telling them we're suing."
Wishart shared the post, writing: "The casual and indifferent refusal by the met police to investigate the latest cash for honours is a dereliction of duty.
"We have written to them to say if they do not give the reasons for a refusal to investigate we will take action to have this judicially reviewed."
The Met Police said on Sunday: "The MPS received a letter before claim on 19 November and will respond to this letter in due course."
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