Boris Johnson will use a showdown meeting with Theresa May this week to demand reassurances that the Prime Minister will not agree to make substantial payments to the EU after Brexit.

The Foreign Secretary is concerned by reports that Mrs May is preparing to announce that she will carry on paying up to £10 billion per year to the EU during a transition period, which could be as long as three years.

He used a Telegraph article on Friday to insist that Britain should only pay “what is due” and should pay nothing to access the single market.

Mr Johnson has been told he will not be sacked over the article, which had not been authorised by Downing Street and was regarded by some as a deliberate attempt to undermine Mrs May.

However, there are fears among some Tory MPs that Mr Johnson could take the “catastrophic” decision to ­resign within days if Mrs May fails to appease him in a key speech on Brexit to be given in Florence on Friday.

Senior Conservatives are nervous that the maverick Mr Johnson could “explode the fragile unity of the party” by quitting the Cabinet if Mrs May signals she is prepared to pay tens of billions in an EU “divorce” deal. Ministers are also braced for a possible Cabinet reshuffle after next month’s party conference, at which Mr Johnson could be demoted if he does not step into line.