Rishi Sunak will make the case for Ben Wallace to take over as Nato secretary-general when he meets Joe Biden tomorrow.

The Defence Secretary is seen as a strong contender for the soon-to-be vacant role leading the military alliance, replacing Jens Stoltenberg who will retire in the October.

Mr Sunak is expected to encourage the US president to back Mr Wallace for the top job during their talks at the White House.

A defence source said that the Prime Minister's bilateral discussions in Washington would be 'critical' to Mr Wallace's chances of success.

'The reality is that the decision about the Nato job is taken in the White House and the [US] Department of Defense,' the source said.

'There is no formal process or show of hands, it is all about reaching a consensus that the Americans are happy with.

'So for every candidate it is critical that their case is made to the Americans at the highest level, and it is notable how warmly the PM has been talking about Ben's qualities this week.'

The source said Lithuanian Prime Minister Ingrida Ðimonytë is viewed as favourite for the job, but predicted that some Nato states may view it as too provocative to pick a candidate from a country bordering Russia, giving Mr Wallace a potential opening.

Eastern European countries have never been represented in the role.

Denmark's Prime Minister Mette Frederiksen - who met Mr Biden in Washington on Monday - is also thought to be in the running, and would become Nato's first female chief if chosen.

Earlier this week Mr Sunak praised the Defence Secretary's record and the UK's contribution to Nato.

'Ben does a fantastic job. He is a great Defence Secretary,' Mr Sunak told reporters.

'This is a conversation that is happening amongst leaders around the world more generally and I'm sure it will continue to happen.

'Ben is widely respected among his colleagues around the world. We're one of the only countries that participates in every single Nato operation. We are widely perceived as a thought leader in Nato.

'And I think if you asked the Nato secretary general, he would agree with everything I've just said and we will always continue to be a strong contributor and participant in Nato.'

Last month Mr Wallace appeared to publicly show an interest in the Nato post, admitting it was 'a job I'd like'.