document: The claim: "We are giving £20bn a year or £350m a week to Brussels."
  Reality Check verdict: We are not giving £20bn a year or £350m a week to Brussels - Britain pays £276m a week to the EU budget because of the rebate.
  He has gone with that figure despite a letter from the head of the UK statistics watchdog Sir Andrew Dilnot on Thursday, which described it as "potentially misleading".
  The figure used is roughly the UK's gross contribution to the EU - the amount that would be contributed to the EU Budget were it not for the rebate.
  We say "roughly the UK's gross contribution" because the annual figure that Boris Johnson used was £20bn, somewhat higher than the £18.8bn figure for 2014 - the most recent figure available - while the £350m a week figure is a bit below the actual figure of £361m.
  The important point is that it's not the sum we send to Brussels because the rebate is deducted before any money is paid. In 2014, the UK's contribution to the EU budget - the amount we paid to Brussels - was actually £276m a week.
  If Britain were to give up the rebate, then the UK would have to pay the gross contribution, and it should be noted that the rebate is not a permanent feature of Britain's membership of the EU.
  But we do not pay the gross contribution at the moment, the rebate is safe until 2020, and Britain has a veto over the process that would remove or reduce it.
  Sir Andrew Dilnot is particularly concerned about suggestions from Vote Leave that a sum of money equivalent to the gross contribution could be spent on other priorities such as the NHS, when some of it is not spent at all (the rebate) and some of it is currently used to support UK farmers, for example.
  READ MORE: The facts behind claims in the EU debate
summary: Mayor of London Boris Johnson used the figure in The Sun this morning that the UK gives £350m a week to Brussels.