Exclusive: Letter from PM's lawyers says there is no public interest for an investigation into his links with the US businesswoman.
The entrepreneur said she is being used as "collateral", but avoided answering questions over allegations she had an affair with the-then London mayor.
The private prosecution was launched after "misleading" claim that the UK sends £350 million a week to the EU.
Vichai Srivaddhanaprabha's son arrived at the King Power Stadium on Monday to pay tribute to his father.
Topshop boss denies claims of unlawful sexual or racist behaviour.
The retail magnate, who denied the allegations against him, is facing fresh calls for his knighthood to be stripped.
Lord Hain used parliamentary privilege.