Britain is to establish joint diplomatic missions abroad with Canada, the Foreign Secretary William Hague will announce.
Mr Hague is to reveal details of this planned new co-operation when he meets his Canadian counterpart John Baird in Ottawa on Monday.
The agreement for joint diplomatic missions overseas could eventually involve Australia and New Zealand.
Mr Hague says it will give Britain "a bigger reach abroad for less cost".
Canadian Foreign Affairs Minister John Baird said it was no more than an "administrative" arrangement, but some former diplomats criticised the plan.
"[Britain] happens to be our old colonial master," retired Canadian envoy Chris Westdaltold the Canadian Press. "In that situation, we have to be hypersensitive to the notion that people will see we're not really quite free, are we?"
The aim is to expand the countries' diplomatic presence in places where either London or Ottawa does not already have an embassy.
Mr Hague has issued a statement ahead of his announcement on Monday, which said: "As David Cameron said when addressing the Canadian parliament last year: 'We are two nations, but under one Queen and united by one set of values'."
"We have stood shoulder to shoulder from the great wars of the last century to fighting terrorists in Afghanistan and supporting Arab Spring Nations like Libya and Syria. We are first cousins.
"So it is natural that we look to link up our embassies with Canada's in places where that suits both countries. It will give us a bigger reach abroad for our businesses and people for less cost."
The BBC's diplomatic correspondent Jonathan Marcus says that, while money is one factor driving co-operation, this diplomatic link-up will also play well in euro-sceptic circles within the Conservative Party, where there is a good deal of unease at the expansion of the European Union's own diplomatic corps - the External Action Service.