The former chancellor wrote to the prime minister to demand details as MPs gear up to seize control of the Commons and pass laws to block a no-deal Brexit on October 31.
Hammond said he and 20 other potential rebels were planning to press Johnson for the information in a meeting on Monday which was cancelled by the PM.
He urged Johnson to reveal what he has proposed to the EU to replace the Irish border backstop.
Downing Street was urged to publish the plans and any response from Brussels, as well as details of negotiation meetings in the last 11 days, before the Commons sits at 2.30pm tomorrow.
Suggesting it was Johnson’s last chance to avoid a major rebellion and likely defeat in the Commons on Tuesday, Hammond wrote: “I am sorry that you have decided not to go ahead with the arranged meeting today between yourself and the group of 21 colleagues who wrote to you seeking reassurances about your Brexit negotiating strategy on August 12.
“Since many colleagues were intending to use your responses in the planned meeting today to decide how they, personally, should proceed over the next few days, it would be extremely helpful if you could respond to these specific questions before the House sits tomorrow.”
Johnson has threatened the group with expulsion from the party if they rebel on Tuesday.
But several, including David Gauke and Rory Stewart, have signalled they are ready to pay the price in order to back a cross-party plan to block a no-deal Brexit.
On Tuesday they are set to join opposition parties in a move to take over Commons business and pass legislation this week that would aim to make a no-deal Brexit on October 31 impossible.
Hammond yesterday rejected one-on-one talks with Johnson after the prime minister pulled out of a meeting with backbench rebels.
Earlier, a senior government source said Johnson’s Brexit ‘sherpa’ David Frost would be in Brussels later this week to hold talks with EU counterparts.
They will focus on replacing the Irish backstop, as well as changes to the non-binding political declaration to make it “much clearer” that the future EU-UK relationship would be a “best in class free trade agreement”.
Speculation is rife in Westminster that Johnson could call an election if he loses Tuesday’s vote.