By Hester Plumridge 

LONDON--Three days after AstraZeneca PLC slammed the door on Pfizer Inc.'s latest $120 billion takeover offer, some big investors in the British drugs giant are refusing to give up.

AstraZeneca's sixth-largest investor, Legal & General Group PLC, is urging the company's board to reverse its rejection of the deal, two people familiar with the situation said. The move is the most recent by a still-small number of investors--holding 8.4% of the company's shares--whose opposition to the deal has become public. Underscoring the investor divide over AstraZeneca's decision, a handful of investors, representing 9.6% of shares outstanding, have backed the board publicly.

The investment arm of the U.K. insurer holds a 3.5% stake in AstraZeneca. It wrote to the company's board asking it to engage in talks with Pfizer, one of these people said. Legal & General declined to comment. AstraZeneca declined to comment on individual shareholder interactions, but said it is continuing to engage in general with its investors.

AstraZeneca on Monday rejected a "final" takeover offer from Pfizer worth GBP55 ($92.89) a share. Under U.K. takeover rules, the companies have until Monday to enter talks, or the offer will expire. The quick rejection dramatically dimmed hopes of a deal, though an investor uprising could force the company's hand and convince it to re-engage in talks.

After falling sharply Monday after the rejection, AstraZeneca shares closed 2.59% higher Wednesday in London trading, after rising close to 4% at one point, after The Wall Street Journal reported Legal & General's opposition. Still, at GBP44.20 late Wednesday, shares are still trading far below Pfizer's offer price, suggesting most investors still don't believe a deal can be done.

Legal & General joins AXA SA, Schroders PLC and Jupiter Fund Management PLC as large AstraZeneca investors that have expressed disappointment at its rejection of the offer. Schafer Cullen Capital Management Inc., a New York investment firm that says it controls around 1.5% of AstraZeneca stock in accounts and funds managed on behalf of its clients, told AstraZeneca Chairman Leif Johansson on a half-hour telephone call Wednesday that it was "unhappy," and that AstraZeneca should sit down with Pfizer, according to Jennifer Chang, a Schafer Cullen portfolio manager. Combined the five investors hold about 8.4% of AstraZeneca stock.

"Many shareholders--but not necessarily all--will find this an attractive offer," said Jim Stride, head of U.K. equities at AXA Investment Managers U.K., in a statement issued Wednesday. "Accordingly we believe that the board...was arguably wrong and acted too hastily to dismiss the latest proposal from Pfizer."

Still, odds that enough shareholders will challenge the board's decision are steep. Other big, influential investors--including Investor AB, Aberdeen Asset Management PLC, Fidelity Investments Inc., St. James Place PLC, M&G Investments and Threadneedle Asset Management Ltd.--have publicly backed the board. Together these six investors hold about 9.6% of AstraZeneca.

A spokeswoman for Threadneedle said Wednesday the company continued to support the board's stance. "We feel the full implications of the proposed acquisition haven't been sufficiently understood and addressed by Pfizer," she said.

Many other investors, including top-five shareholders BlackRock Inc. and Wellington Capital Management Inc., have declined to comment on their views of the board's actions. Because of U.K. takeover rules, Pfizer's offer will expire by 5 p.m. BST Monday--a holiday in Britain and the U.S. That gives opponents of the board's decision just two more working days to make their case to the board.

If the companies don't enter talks by Monday, Pfizer must wait between three and six months if it wants to return with another bid. Under U.K. takeover rules, Pfizer is permitted in only limited circumstance to change the terms of its offer before Monday.

It can increase the cash component, boosting it from the current 45% cash mix, or add a contingent-value right--a structure in which AstraZeneca could get additional shares or cash in the event of an experimental drug being approved. However, it isn't allowed to increase the value of its total offer above GBP55.

Although it is possible that shareholders might pressure the AstraZeneca board into engaging with Pfizer, or that Pfizer might fiddle with components of its offer, "it seems increasingly likely that there will be no further developments for 3-6 months beyond the 26 May," analysts at Barclays wrote in a note to clients Wednesday.

-Jonathan D. Rockoff contributed to this article.

Write to Hester Plumridge at Hester.Plumridge@wsj.com

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