Keir Starmer has vowed to “choke off” Russian revenue with further sanctions and to provide hundreds of millions of pounds worth of energy support for Ukraine, as he met world leaders in France for the G7.

After a torrid political week at home, the British prime minister sought to put himself on the front foot on the international stage at the meeting of the group of seven, which kicked off on Monday in the French spa town of Évian-les-Bains, on the shore of Lake Geneva.

Starmer is expected to meet the Ukrainian president, Volodymyr Zelenskyy, and the prime minister of India, Narendra Modi, on Tuesday, the first full day of the summit.

He will announce sanctions against Russia, days after British troops seized a Russian shadow fleet oil tanker in the Channel.

He is also set to use the meeting to reassure Donald Trump he is willing to raise defence spending, after the resignation of his defence secretary, John Healey, last week and ongoing delays to his defence investment plan – but is not expected to have a bilateral meeting with the US president.

The defence investment plan is now expected to be published before the Nato summit in Ankara in Turkey, which begins on 7 July.

After a series of devastating Russian attacks on Ukrainian power infrastructure, Starmer has pledged £210m for Ukraine’s nuclear plants for the next two years, which he said would “power Ukraine through the winters ahead”.