"The prime minister is coming tomorrow," spokesman Lukasz Jasina told AFP on Wednesday when asked about Johnson's plans.

Jasina said Johnson would meet with his Polish counterpart Mateusz Morawiecki and probably also with British troops in the country.

Britain has already deployed around 100 troops -- military engineers -- to Poland to help guard the border after thousands of migrants tried to cross over from Belarus.

The West accused the Belarusian regime of encouraging the migrants -- most of them from the Middle East -- to cross as retaliation against sanctions in a form of unconventional "hybrid" warfare.

Belarus, a close Russian ally, denied this.

British Defence Secretary Ben Wallace on Monday said the additional troops were being sent in a "bilateral deployment to show that we can work together and send a strong signal that Britain and Poland stand side by side".

In recent weeks, Russia has sent tens of thousands of troops to its border with Ukraine and into parts of neighbouring Belarus, sparking fears in the West of a possible invasion of Ukraine.

Russia has denied having any plans to invade and has accused NATO countries of using belligerent rhetoric and having an aggressive posture in ex-Communist European countries.