British troops deployed as part of a multinational mission to secure any future peace in Ukraine would be permitted to fire back at Russian forces if attacked, Western officials have said, underlining the high stakes of the potential operation led by Britain and France.

The proposed deployment, named the Multinational Force-Ukraine (MNF-U), could only take place once Russia's war in Ukraine has stopped and peace has been established. The force would enter the country at the invitation of the Ukrainian government.

Whether the operation ever happens remains in serious doubt. A cessation of hostilities would likely involve conditions and compromises agreed between Kyiv and Moscow in the absence of an outright victory by either side, and the Kremlin has repeatedly ruled out the idea of troops from NATO nations helping to secure a lasting peace. President Vladimir Putin has rejected such a prospect in the past, yet London and Paris are pressing ahead with their preparations.

In the biggest demonstration of that planning so far, around 300 British troops and more than 2,000 soldiers from 10 other partner nations are taking part in an exercise in Poland that began on Monday and will last for almost a fortnight. The troops are rehearsing how they would work together in the various roles they might perform in any future operation.

Those roles include deploying fast jets to help secure Ukraine's skies and encourage commercial airliners to resume flights. The country's airports have been closed to civilian traffic since Russia's full-scale invasion began in February 2022. At sea, Turkish demining warships would be deployed to make Ukrainian maritime routes safer for commercial vessels, while land forces would focus on training Ukrainians to regenerate the country's fighting strength. Because Ukraine fields the most battle-hardened troops in Europe, its soldiers will also share their knowledge with British, French and other European trainers.

The other nations joining the Poland exercise, dubbed Exercise Koalicja, are Belgium, Estonia, Germany, Lithuania, the Netherlands, Poland, Spain, Sweden and Turkey. The United States will not be present and is not part of the multinational force, although separate US security guarantees for Ukraine are seen as a vital element of any credible peace deal. A Western official said the force will not rely on US communications or other systems during the exercise.

The Ministry of Defence said the training "will test how to move, protect and supply a force that would help rebuild Ukraine's Armed Forces once the fighting ends". The British contingent includes soldiers from the British Army's 7 Brigade, known as the Desert Rats.

Defence Secretary Wes Streeting said: "This exercise shows the UK is stepping up, alongside France and our allies, to prepare for the day peace comes to Ukraine. This isn't a hypothetical plan, it's a real force, ready to help Ukraine once the fighting stops."

Britain has stressed that the operation is not a NATO mission and will not undertake peacekeeping or ceasefire-monitoring duties. The force is commanded by Lieutenant General Tom Bateman and forms the military wing of what the UK and France call the coalition of the willing in support of Ukraine. "The MNF-U will be more ready every day to support Ukraine when the fighting ends, ensuring the Armed Forces of Ukraine are in the strongest possible position to deter future aggression," Bateman said.