Britain is expected to supply Storm Shadow missiles for use by Ukraine on targets inside Russia, now that the US president, Joe Biden, has agreed to do the same for the similar American long-range Atacms weapon.

Keir Starmer, the prime minister, said at the G20 summit that the UK recognised it needed to “double down” on its support for Ukraine, while diplomatic sources briefed they expected other European countries to follow the US lead.

The prime minister said that, while he was “not going to get into operational details”, he recognised the need to do more to help Ukraine, whose electricity network was seriously damaged by a wave of Russian bombing on Sunday.

“I’ve been really clear for a long time now, we need to double down. We need to make sure Ukraine has what is necessary for as long as necessary, because we cannot allow Putin to win this war,” the prime minister said.