Senior government figures have said that the UK does not have the capacity to renegotiate dozens of trade deals that already exist between the EU and third countries.

Instead, they are planning to draw up copycat deals to those that already exist, in an attempt to replicate an agreement struck by Theresa May in Japan last week. May and the Japanese prime minister, Shinzo Abe, signed a joint statement that committed their countries to working towards an economic partnership agreement (EPA) as an “immediate priority”.

The statement said: “As the UK exits the EU we will work quickly to establish a new economic partnership between Japan and the UK based on the final terms of the EPA.”

May hopes such instant copycat deals – termed “cut-and-paste Brexit” by opponents – could be reached with other nations to help secure continuity and confidence.

But a senior government source admitted that it was also a question of need for the UK, given the lack of resources to renegotiate the dozens of deals already signed between the EU and third countries.

“We can’t do 40 FTAs, we haven’t got the capacity to do that,” they said.

Darren Jones MP, a leading supporter of Open Britain, said: “The promise of new trade deals with countries around the world is starting to look like yet another broken Brexit promise. Given we have not negotiated a single trade deal for forty years, it’s hardly surprising that the government lacks the capacity to take on 50 at a time.

“Ministers should focus on our most important trade deal – the one with the European Union – and ensure that half of all our trade remains truly free by seeking to negotiate for continued British membership of the single market and customs union.”

The Liberal Democrat leader, Vince Cable, said the government’s intention to look for copycat deals was an example of “yet another Brexit broken promise”.

“First we learned that countries such as Japan and India have no interest in doing separate trade deals with the UK. Now we learn that even if there was appetite for such deals, we wouldn’t have the capacity to negotiate them,” Cable said.

“The government is clearly completely underprepared and is presiding over a Whitehall farce. This is like a nightmare divorce, where one partner decides to walk away with no idea of what they will move on to.”

In Japan, the positive agreement came amid some clear concerns about the process by which the UK will leave the EU. Abe said that Britain was a “gateway to the EU” for many Japanese companies.

“With this in mind, I have asked Prime Minister May for her continued consideration in ensuring transparency and predictability so as to minimise its impact on business activities, including Japanese companies,” he said.

“Our country would like to minimise the impact to companies because of Brexit, in the process of negotiations for Brexit. I hope that transparency and predictability will be ensured. That would be very important.”

May promised to “listen to the voices of Japanese corporations” calling for a “smooth and orderly” Brexit.