Polish Prime Minister Donald Tusk and British Prime Minister Keir Starmer are expected to sign the agreement during talks in the British capital.
Speaking before departing for London, Tusk said Poland wanted to bring relations with Britain "to the highest possible level," with a particular focus on security and defence against Russia.
The British government said the treaty would deepen defence cooperation in response to increasingly hostile threats across Europe, following similar agreements Britain has signed with France and Germany.
According to London, the pact is expected to strengthen border security, combat organised crime and deepen defence cooperation between Britain and the European Union.
Tusk and Starmer are also expected to discuss what officials described as a sharp increase in hybrid threats, including cyberattacks, espionage and suspected Russian-linked sabotage operations across Europe.
Poland has repeatedly warned that its role as a hub for military assistance to Ukraine has made it a target for Russian cyberattacks, espionage and disinformation campaigns.
Tusk said on Tuesday that Warsaw had sought provisions designed to "genuinely increase Poland's security in peacetime, so as to prevent potential threats."
He added that the two countries were already cooperating closely on support for Ukraine, which he said provided an additional reason to strengthen bilateral security ties.
The treaty builds on defence cooperation agreements signed by Poland and Britain in 2017 and 2023, Polish state news agency PAP reported.
It is expected to include a mutual defence commitment similar to recent British agreements with France and Germany.
Beyond defence, the agreement will also cover economic resilience, energy security, illegal migration and organised crime.
Tusk compared the accord to a treaty Poland signed with France in Nancy last year, which included a mutual military assistance clause as well as cooperation in the defence industry, the economy, agriculture and science.
He said such agreements, alongside NATO, the country's alliance with the United States and the EU's SAFE defence financing programme, formed the basis of Poland's broader security strategy.
Poland is also holding talks with Germany on expanding bilateral cooperation, including in defence, Tusk added.
Starmer said the treaty with Poland marked a major strengthening of ties between the two countries.
"Britain and Poland are already close allies and friends, but the challenges Europe now faces demands an even stronger partnership," he said, as quoted in a statement.
"This treaty is the biggest step forward in our defence and security relationship with Poland in a generation, allowing us to confront modern security threats that may be less visible but no less dangerous," Starmer added.
The British government said the agreement was expected to pave the way for closer cooperation in developing next-generation weapons systems and strengthening European defence manufacturing capabilities.
That cooperation is expected to include work on new air defence systems and the joint development of "a next-generation medium-range air defence missile."
Poland and Britain also plan to expand cooperation on uncrewed military systems to reinforce NATO’s eastern flank through what London described as "next-generation land capabilities."
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Source: IAR, PAP gov.uk