Britain has just six weeks to finish negotiating a post-Brexit deal with Japan if it wants an agreement ratified this year, Tokyo's chief negotiator has warned in an interview. 

Speaking to the Financial Times, Hiroshi Matsuura said the short timeline would mean both sides have to "limit their ambitions", suggesting goals set by London might be unachievable.

"To avoid a gap in January, we must pass this in the autumn session of the Diet," Matsuura told the paper, referring to the Japanese parliament. 

"That means we must complete negotiations by the end of July. The shortage of time means that both sides will have to limit their ambitions," he added.

Any deal is expected to be based on a broad EU-Japan trade agreement that came into effect last year, but London has signalled it hopes to win the reduction or elimination of additional Japanese tariffs. 

Britain is scrambling to negotiate post-Brexit trade deals, including in acrimonious talks with the EU, which last week brushed aside calls from British Prime Minister Boris Johnson for an agreement by July. 

Britain officially completed its delayed departure from the bloc on January 31. 

It now has until the end of December to strike deals with the EU and other trading partners.