Ghana's national oil company is threatening to scupper ExxonMobil's bid to buy a stake in the country's Jubilee field, the largest deepwater oilfield in Africa.

The national oil company is pushing Cnooc, the Chinese national oil company, to make a counterbid for Kosmos, the small US-based company that ExxonMobil last week decided to acquire by signing an exclusive binding agreement worth more than $4bn (€2.7bn, £2.5bn).

Kosmos, which discovered the oilfield during the previous Ghanaian presidency, has fallen out of favour with the government, according to industry insiders. It is backed by Blackstone and Warburg Pincus, the private equity groups, which stand to generate a fourfold return on the deal.

GNPC's decision runs the risk of undermining the reputation of Ghana as one of west Africa's more stable and transparent countries for investors, analysts said. Robin West, chief executive of PFC Energy, the consultancy, said: “The Ghanaians run a serious risk if they disrupt this sales process. It could delay projects and chill investments.”

Several international and national oil companies bid for Kosmos. However, Cnooc was not among them, people close to the Chinese company said. Instead, Cnooc conducted parallel talks with GNPC, which hired Morgan Stanley to advise it. Meanwhile, the sale of Kosmos was delayed by several months.

Joseph Oteng-Adjei, Ghana's energy minister, told the Financial Times at the time he had not wanted to block the sale process and that he had wanted to protect Ghana's interests “without cheating or deceiving anyone who has benefited the country by investing in our oil industry”.