The British Ambassador to Ireland has suggested that too many countries in the West have been less than honest with themselves over the past decade about the nature of the challenge posed by Russia and other autocratic states.

Paul Johnston said the international structures developed after World War II and evolved after the Cold War had failed to deal with the challenge posed by Russia and had failed Ukraine.

Calling for the imposition of tougher sanctions, Mr Johnston said imports of Russian oil and gas must be stopped.

He described Ukraine's victory as "a strategic imperative", adding that if Russian President Vladimir Putin wins it would encourage aggressors everywhere.

Mr Johnston was speaking as part of the Jean Monnet Lecture Series at University College Cork. He said his lecture was a personal reflection and not necessarily an expression of British government policy.

"The last ten years have seen perhaps too many in the West being less than honest with themselves about the nature of the challenge we have faced from Russia and other autocratic states," Mr Johnston said.

"The fate of Ukraine remains in the balance. We need to go further, including by being even tougher on sanctions. We must cut off oil and gas imports so Putin has nowhere to go to fund his appalling war. And we must put in a place robust and sustained humanitarian response to support the Ukrainian people."

Ambassador Johnston said while his government chose not to seek a formal structured treaty relationship with the EU in the area of foreign and security policy during negotiations leading up to Brexit, it had always made clear that Britain would work closely with the European Union, its institutions and its member states in pursuit of common interests.

"What the Ukraine crisis has shown us, as indeed did the Kosovo crisis a quarter of a century before, is that the interests and values which unite us as Europeans and indeed across the political West, are more important than the issues that can sometimes appear to divide us," he said.

He described Ireland as a key partner of Britain, "listed in our government's integrated review at the top of our priority list, with the US, France and Germany".

"I hope that this crisis will enable us all to recognise the necessity of working together to defend and promote the values we all share," he said.

Mr Johnston said inaction can be the greatest provocation to aggression.

"Russia's invasion of Ukraine - and indeed China’s growing assertiveness - have destroyed the idea that economic integration alone can drive political change or that countries will naturally evolve towards democracy and human rights without help or indeed protection," he said.

Mr Johnston was appointed British Ambassador to Ireland in September 2020.

He previously served in Paris and New York and has held a wide range of political and security roles in the Foreign and Commonwealth Office in London. From 2017 to 2020, he was UK Ambassador to the EU for Political and Security Affairs.