In a surprise announcement, Prime Minister Theresa May called for an early general election on June 8 as Britain prepares for delicate negotiations on leaving the European Union by 2019.
The political calendar was already heavy in 2017 with upcoming votes in France and Germany having major implications for the eurozone. The first round of the French presidential election is scheduled for Sunday.
"The market dislikes uncertainty," said Yutaka Miura, a senior technical analyst at Mizuho Securities.
"Investors had already been a little on edge over the French presidential election, and now the UK general election is creating further uncertainty in Europe," he told Bloomberg News.
Tokyo's benchmark Nikkei 225 inched up 0.07 percent, or 13.61 points, to close at 18,432.20, while the Topix index of all first-section issues ended flat, slipping 0.01 percent, or 0.11 points, to 1,471.42.
Jittery traders bought the yen, which is widely seen as a safe investment but a stronger currency is bad news for Japanese shares as it dents exporters' profitability.
The dollar was changing hands at 108.69 yen, slightly up from late in New York but weakening from 109.05 yen in Asia on Tuesday.
News of the British election pushed up the pound to $1.2905 at one point, its highest level since October 2016, on hopes that it could result in a strong mandate for May and boost her negotiating power with the EU. In Asia, sterling fetched $1.2812.
Shares of Hitachi, which has operations in Britain, dropped 0.53 percent to 576 yen while Nissan, which has two plants there, edged up 0.04 percent to 1,019.5 yen.
Bank Mitsubishi UFJ Financial fell 0.07 percent to 655.4 yen while Sumitomo Mitsui Financial rose 0.80 percent to 3.878 yen.
Toshiba jumped 4.87 percent to 208.8 yen after Japan's Nikkei business daily reported the loss-hit industrial giant plans to spin off infrastructure and other major operations into separate companies to quicken management decisions.
Separately, broadcaster NTV, citing an internal document sent by the firm's president, reported that Toshiba was considering laying off some 1,000 employees.
According to the Asahi newspaper, Japan's industry ministry is considering having government-backed firms make a joint bid for Toshiba's memory chip business with US semiconductor company Broadcom, amid concerns about the sensitive technology going abroad.