Boris Johnson’s leadership has been thrown into turmoil amid growing calls from backbenchers for him to resign.
If this threshold is reached, Conservative MPs will vote on whether the Prime Minister should continue in office.
Half of Conservative MPs would have to vote against the Prime Minister for him to face a leadership contest.
Eighteen Tory MPs have so far confirmed that they have written to Sir Graham, while 20 have publicly questioned Mr Johnson’s leadership.
More than half of the MPs who have come out against the PM represent London, the south east or south west.
Five London MPs - Sir Bob Neill (Bromley and Chislehurst), Stephen Hammond (Wimbledon), Elliot Colburn (Carshalton) David Simmonds (Ruislip, Northwood and Pinner) and Nickie Aiken, have so far called on Mr Johnson to stand down or hold a confidence vote.
Tory MPs who have said PM should resign or have publicly criticised him over partygate:
Elliot Colburn, MP for Carshalton (letter confirmed)
Stephen Hammond, MP for Wimbledon (letter confirmed)
Bob Neill, MP for Bromley & Chislehurst (letter confirmed)
David Simmonds, MP for Ruislip, Northwood and Pinner
Nickie Aiken, MP for Cities of London and Westminster
Peter Aldous, MP for Waveney (letter confirmed)
Steve Brine, MP for Winchester and Chandler (letter confirmed)
Sir Roger Gale, MP for North Thanet (letter confirmed)
Caroline Nokes, MP for Romsey and Southampton North (letter confirmed)
Nick Gibb, MP for Bognor Regis and Littlehampton (letter confirmed)
Steve Baker, MP for Wycombe
John Baron, MP for Basildon and Billericay
Tim Loughton, MP for East Worthing and Shoreham
Angela Richardson, MP for Guildford
Dan Poulter, MP for Central Suffolk
Tom Tugendhat, MP for Tonbridge and Malling
Paul Holmes, MP for Eastleigh
Dame Andrea Leadsom, MP for South Northamptonshire
Gary Streeter, MP for South West Devon (letter confirmed)
Tobias Ellwood, MP Bournemouth East (letter confirmed)
Mark Harper, MP for Forest of Dean (letter confirmed)
Anthony Mangnall, MP Totnes (letter confirmed)
Anne Marie Morris, MP for Newton Abbot (letter confirmed)
Aaron Bell, MP for Newcastle-under-Lyme (letter confirmed)
Alicia Kearns, MP for Rutland and Melton (letter confirmed)
Andrew Bridgen, MP for North West Leicestershire (letter confirmed)
Karen Bradley, MP for Staffordshire Moorlands
Nigel Mills, MP for Amber Valley
Andrew Mitchell, MP for Sutton Coldfield
Jeremy Wright, MP for Kenilworth and Southam
William Wragg, MP for Hazel Grove (letter confirmed)
Neil Hudson, MP for Penrith and The Border
Craig Whittaker, MP for Calder Valley
John Stevenson, MP for Carlisle (letter confirmed)
Robert Largan, MP for High Peak
David Davis, MP for Haltemprice and Howden
Andrew Jones, MP Harrogate and Knaresborough
Julian Sturdy, MP for York Outer