The world's leading index service providers are vying with each other to further tap into the Chinese mainland's market with faster mapping.

New York-based global index business MSCI said in a statement released on its website on Tuesday that it had proposed to increase the A-share market's large-cap securities from the current 5 percent to 20 percent. It will happen in two phases in line with MSCI's index reviews in May and August 2019.

Meanwhile, MSCI also proposed to add Shenzhen's startup board ChiNext shares to the list of eligible segments for inclusion starting from the May 2019 semi-annual index review. It is also considering adding Chinese mid-caps with a 20 percent inclusion factor in one phase as part of the May 2020 semi-annual index review.

It is less than one month after MSCI's second stage of its partial inclusion of A shares in late August. The first step of partial inclusion was realized in June.

Meanwhile, FTSE Russell will announce its decision concerning the inclusion of the Chinese mainland's A shares early on Thursday morning. The London-based index service provider's CEO Mark Makepeace said earlier this month that it would offer a heavier weighting for A shares.

MSCI is the world's largest index service provider. It manages about $10.5 trillion. About 60 percent of the world's passive funds that track global and emerging market indexes are based on the MSCI index, while the remainder refers to its London-based rival FTSE Russell.

The most important reason for the overseas institutions' competition for the A-share market is "its investment value which was hardly seen before", said Jiang Qijia, senior analyst of the research department at financial service provider Noah Holdings Ltd. As he further explained, the A-share market's price earnings ratio was 14.5 times by Sept 21, while its price-to-book ratio was 1.6 times.

"Overseas investors are mostly institutional investors. The comparatively low market cap of the A shares is the main attraction," he said.

Meanwhile, capital inflow into the A-share market via the stock connect mechanism between the Chinese mainland and Hong Kong is also an indicator of the overseas investors' enthusiasm, said Jiang.

Although the benchmark Shanghai Composite Index dropped by 16 percent since the beginning of this year, northbound capital inflow into the Chinese mainland exceeded 230 billion yuan ($33 billion). At present, foreign investors hold about 6 percent in A shares in general, which is equal to the ratio held by public funds, he added.

"MSCI and FTSE Russell are the most influential indexes in the world. There is also competition between the two. Once FTSE Russell decides to include A shares, MSCI will be stimulated to give a heavier weighting to A shares," he said.

Buoyed by the overseas institutions' increased interest in the Chinese mainland market, the A-share market rallied on Wednesday. The Shanghai Composite Index went up 0.92 percent to close at 2806.81 points, while the Shenzhen Component Index climbed 0.8 percent to close at 8420.54 points.