SHANGHAI/BEIJING, July 3 (Reuters) - China's finance ministry on Wednesday asked underwriters of this week's 30-year treasury auction to resubmit their bids taking into account the central bank's new bond borrowing plans, according to seven people familiar with the matter.

The unusual request is a possible signal the People's Bank of China is about to step in to the market and it shows how keenly authorities are focused on the effects of its trading.

The People's Bank of China (PBOC) announced in May it would trade in treasury bonds for the first time in 17 years as a policy tool, as a mountain of money that's stuck in the banking system drives a runaway rally in the bond market.

On Monday, the PBOC said it would soon borrow sovereign bonds from primary dealers in a likely prelude to selling.

On Friday, the Ministry of Finance will auction 58 billion yuan of 30-year treasury bonds and underwriters told Reuters they were asked on Wednesday to resubmit bids.

The ministry said in a notice to underwriters that "demand from some underwriters may change" following the PBOC's bond borrowing announcement, according to the sources.

Underwriters include primary dealers, brokers and other institutions, and promise to buy bonds at auction.

It was not clear what changes to their bids were expected by the ministry or whether any were made. The ministry did not immediately respond to Reuters' request for comment.

The number of underwriters at the auction is much larger than the pool of primary dealers. The outcome of the sale will be closely watched as it is the largest tranche of sovereign debt on offer since sales of special ultra-long bonds in May.

Bond prices, which plunged on Monday when the PBOC unveiled its borrowing intention, were recovering for a second straight session on Wednesday.

Thirty-year treasury futures for September delivery rose more than 0.2% on Wednesday morning and have recouped much of Monday's losses. Yields on 30-year government bonds , which move inversely to prices, fell roughly 0.4 basis points to 2.456%.

"We think the logic of bond yields trending lower is not fundamentally changed, against the backdrop of weak economic recovery and loose monetary policies," Guolian Securities analyst Li Qingqi wrote in a note to clients.

($1 = 7.2731 Chinese yuan) (Reporting by Shanghai and Beijing newsroom; Editing by Christopher Cushing and Sam Holmes)