Big-ticket M&A slowed down significantly in the Asia-Pacific region in the first quarter of 2017.

According to league tables published by Mergermarket, Bloomberg and Thomson Reuters, M&A deals during the first three months of this year declined both by deal count and volume.

While most of Asia saw a drop, four Indian firms made the top 10 legal advisers for M&A deals by volume, thanks to the $12.7bn sale of Vodafone Group's Indian business to Mumbai-based carrier Idea Cellular, the largest M&A deal announced in the first three months of 2017.

The quartet that played lead roles on the deal are: S&R Associates, AZB & Partners, Vaish Associates, and Bharucha & Partners.

Slaughter and May, which is also acting for Vodafone, topped both Bloomberg and Thomson Reuters' rankings for M&A deals by deal volume. Mergermarket ranked Slaughters second after King & Wood Mallesons. The rest of the top 20 varies greatly among the three organisations' league tables, as different deals were included based on different methodologies.

Despite the difference in rankings, all three reported a slowdown in Asia-Pacific M&A deals during the first three months of this year. According to Mergermarket, the Asia-Pacific region excluding Japan saw 697 deals announced in the first quarter, worth a total value of $124.8bn – 120 fewer deals by deal count than the same period last year. And the value is down by $14.5bn, or 10.4%, compared with the same period last year.

Bloomberg reported a 14.3% drop, to $174.2bn in total deal value, the lowest since the third quarter of 2014. Thomson Reuters, whose rankings included more deals, said the year-on-year deal value decrease reached 31%.

The decline was in part due to a visible drop in Chinese outbound acquisitions, a theme that dominated last year's Asia M&A market. After a record-breaking year of Chinese overseas M&A, the Chinese government imposed a series of measures to curtail capital outflow toward the end of 2016, in order to stabilise its currency's exchange rate against the dollar and other foreign currencies.

Meanwhile, the US government has vowed to adopt tougher regulatory scrutiny of Chinese-led acquisitions, especially in areas where national security might come into play. Measures from both sides have resulted in Chinese investors retreating from overseas deals this year.

Mergermarket reported that 75 Chinese outbound deals with a total value of $11.8bn were announced in the first quarter of 2017, a steep drop from the 96 deals worth $82bn during the same period of 2016. Last year's total deal value was significantly boosted by ChemChina's $43bn planned acquisition of Syngenta. The deal, first announced in February 2016, received long-awaited approval from the Chinese Ministry of Commerce this week, clearing a major regulatory hurdle.

Still, Bloomberg said the Greater China region is still the most sought after for transactions, accounting for more than half (53%) of all deals in the region. The slowdown in outbound deals also made investors turn to domestic targets. Bloomberg noted that 96% of the Greater China deals it recorded in the first quarter were domestic deals. Mergermarket reported a similar pattern within the region, noting that transactions between two Asian parties accounted for 93% of the total deal value of $124.8bn.

Following a strong 2016, the Japanese M&A market also saw a downturn on the previous year. Thomson Reuters recorded deals worth $30.5bn in total value involving Japanese parties, down 27% year on year and the slowest first quarter since 2013. Six deals were valued at $1bn or more, according to Thomson Reuters data; all but one were outbound deals. Takeda Pharmaceutical's $5.2bn acquisition of cancer drugmaker Ariad Pharmaceuticals was the largest deal announced.

Paul Weiss Rifkind Wharton & Garrison – Ariad's counsel in the Takeda deal – Morrison & Foerster, Simpson Thacher & Bartlett and Sullivan & Cromwell were among the firms leading the rankings of Japan deals.