The Crown Prosecution Service has asked cash-for-honours probe police to "undertake further inquiries".
Assistant Commissioner John Yates has been heading up the inquiry
Scotland Yard handed over its main file in April to the CPS, which will decide whether any charges will be brought.
Police interviewed 136 people while probing whether honours were given to people in return for money.
Four people were questioned under caution, including members of Tony Blair's inner circle. All deny wrong-doing. No-one has been charged.
The investigation began a year ago, after it emerged that secret loans had been made to Labour before the 2005 general election, and that some lenders had subsequently been nominated for peerages.
The inquiry was widened to cover the other main parties. Among those questioned as potential witnesses were Tony Blair and former Conservative leader Michael Howard.
Those arrested and questioned under caution were head teacher Des Smith, who was involved in the government's city academies programme, Labour donor Sir Christopher Evans, Downing Street adviser Ruth Turner and Labour's chief fundraiser Lord Levy.
In February the CPS said Mr Smith would not face any charges.
The police investigation appeared to switch its focus in recent months, from the question of cash-for-peerages to allegations of a cover-up.