Poland and Britain signed a new defence treaty on Thursday as Prime Minister Theresa May sought to deepen ties with Warsaw ahead of her countrys exit from the EU by March 2019.

"Our defence and security cooperation is already strong, but we have gone even further today in signing this landmark joint UK-Poland treaty on defence and security cooperation," May said at a joint press conference with counterpart Mateusz Morawiecki.

"It is only the second such treaty we have signed with a European Union country," May said, adding that "there could be no clearer expression of the closeness of the UK's relationship with Poland."

May said the new deal will cover training, information sharing and defence cooperation.

The countries are NATO allies and British troops are stationed in Poland as part of the Western defence alliance's bid to reinforce its eastern front and deter a more assertive Russia.

May said the two sides also agreed to bolster "cooperation to counter Russian disinformation in the region including through new joint strategic information projects."

"We are both deeply concerned by Russia's attempts to weaponise information. The Kremlin is seeking to undermine the international rules-based system, and it will not succeed."

"The forum will be business-led and will work to identify any remaining barriers to bilateral trade as we prepare to leave the EU," May said.

She said that she wanted to assure the over one million Polish citizens in Britain that "they are a strong part" of British society "and we want them to stay."

Morawiecki, however, said that he wanted those Polish migrants to return to bolster a weak labour market.

Hundreds of thousands of Poles flocked to Britain in search of a better life after Warsaw's 2004 entry into the European Union.

Rule of law

May, however, declined to address the unprecedented disciplinary proceedings the EU launched against Warsaw on Wednesday for judicial reforms that Brussels says threaten the rule of law.

"These constitutional issues are normally and should be primarily a matter for the individual country concerned," May told reporters.

"Across Europe we have a collective belief in the rule of law and I welcome the fact that Prime Minister Morawiecki has indicated that he will be speaking with the European Commission.

"I hope that that will lead to a satisfactory resolution," she said.

Morawiecki said he believed he could convince the EU that far from breaching the rule of law, the highly controversial judicial reforms pushed through by the rightwing government were needed to root out the last vestiges of communism from its justice system.

"I can remind our German partners that after the successful fall of communism in the former east Germany, only 30 to 35 percent of judges passed vetting.

"In Poland, judges weren't vetted. All the judges from the martial law era, the communist and Stalinist eras became judges in free Poland" after the 1989 transition to democracy, Morawiecki said.