By Tetsushi Kajimoto and Takashi Umekawa

TOKYO (Reuters) - Japan's government won't hesitate to respond to excess speculation in the foreign exchange market or conduct unilateral currency intervention to safeguard the export-reliant economy, an adviser to Prime Minister Shinzo Abe said.

Masahiko Shibayama said if markets became excessively volatile, the Bank of Japan could hold an emergency meeting to ease policy further although the scope for such easing remains limited, given interest rates are already in negative territory.

Britain's shock vote to exit the European Union last week caused a spike in the yen, prompting Japanese policymakers to warn investors against pushing safe-haven currency too high, which would deal a blow to exports and policy efforts to defeat deflation.

"We must be on our guard against speculative moves and won't hesitate to take action against excess speculation," Shibayama, Special Adviser to the Prime Minister, told Reuters in an interview.

"Our basic stance is to cooperate with other countries. But that doesn't mean that solo intervention can be ruled out."

While policymakers have threatened intervention, market participants remain sceptical about Tokyo's appetite to intervene in the markets given strong opposition from Washington.

Shibayama described the BOJ's negative rate policy as "powerful medicine", suggesting it might be hard for the central bank to cut rates deeper into negative territory.

The negative rates policy drew criticism from banks for confusing rather than calming markets.

"It caused a bigger than expected impact. Taking that into account, I want the BOJ to carefully consider what it can do" when easing policy further, he said.

(Reporting by Tetsushi Kajimoto; Editing by Sam Holmes)