IDS also owns a highly profitable European parcels business called GLS which made over £300m last year. This offset losses at Royal Mail, allowing IDS to report a small profit.
By bringing GLS parcel know-how to the UK market, coupled with investment in out-of-home delivery lockers, Mr Kretinsky hopes to build a pan-European logistics business.
The hope is that this will enable Royal Mail to claw back market share it has lost in recent years in the profitable and growing parcels business.
Mr Kretinsky has a net worth of £6bn, according to the Sunday Times Rich List.
In addition to owning 27% of West Ham United football club and 10% of Sainsbury's, Mr Kretinsky's companies also own a gas transmission service which still pipes much reduced levels of Russian gas to Europe, paid for and with the consent of the EU.
Earlier this year the takeover was called in for review under national security laws as Royal Mail is considered vital national infrastructure.
But speaking in front of MPs in November, Reynolds had referred to Mr Kretinsky as a "legitimate business figure" whose alleged links to Russia had already been reviewed and dismissed when he became the biggest shareholder in the company nearly two years ago.