A controversial proposal to introduce an EU system for retaining the private data of air passengers faces being stalled at the 11th hour.
MEPs in the civil liberties committee considered the passenger name records (PNR) proposal on Monday and Parliament was due to vote as a whole on Thursday.
But the Liberal group have now tabled an amendment that some MEPs says would "send the whole agreement back to the negotiating table."
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It is believed that amendments have also been tabled by the Greens/EFA and GUE/NGL groups, who have traditionally opposed PNR.
The legislation has been blocked in the past by other parties over privacy concerns but, in the aftermath of the tragic terrorist attacks in Brussels and Paris, the focus has been on stepping up mass surveillance measures, like PNR.
Parliament's rapporteur on the file, Timothy Kirkhope, was particularly scathing of Sophie in't Veld, the Dutch Liberal group lead MEP on PNR who has tabled an amendment to Thursday's vote.
If adopted, Kirkhope, a UK Tory MEP, warns the amendment would "sink" the agreement he helped broker with EU governments last December and "probably lead to the entire negotiation having to start anew."
Speaking on Tuesday, he said, "This is a wolf in sheep's clothing amendment that looks perfectly innocent but is clearly designed to sink the PNR deal. If it is passed the whole agreement between Parliament and European governments would collapse.
"Under the PNR deal all of the relevant PNR data will be shared among EU governments. Those who claim to not want any collection of personal data now seem quite happy to have it scattered across the EU when there is no clear case for it. This amendment is yet another smokescreen.
"MEPs need to understand that any amendment to this agreement would delay it substantially and probably kill the proposal altogether. They need to understand the stakes during Thursday's vote. After five years we have a very good, balanced agreement."
"If MEPs want to vote against it then that is for them to justify, but they should come out and say so, not pretend to be pro-PNR while doing whatever they can behind the scenes to stop it," added the deputy, who serves the European Conservatives and Reformists group home affairs spokesperson.
EPP group Chair Manfred Weber said that his group welcomed the draft law "as an important tool to track and catch terrorists, after it was blocked by other parties over privacy concerns."
The German deputy said, "The legislation is an effective tool in fighting terrorism and organised crime. The European intelligence community, the security services and police authorities across Europe have said time and time again that the EU PNR system is direly needed to fight terrorism.
"The agreement on a data protection regulation strikes a balance between ensuring the privacy of Europeans and encouraging the digital single market and will be a milestone in data protection worldwide.
Green home affairs spokesperson Jan Philipp Albrecht argued that "serious doubts" remain about the legality of the planned agreement.
He said, "The landmark ruling of the European Court of Justice in 2014 made clear that the general retention of data without grounds is not compatible with EU law. This has clear implications for passenger data exchange and retention systems.
"Against this background, it would be irresponsible to proceed gung-ho with a PNR exchange system that goes in this direction and the proposed system must be legally assessed."
"There is also no evidence introducing a large and blunt data dragnet will improve security and could prove counter-productive by diverting resources badly-needed elsewhere, such as for targeted surveillance."
This week's PNR vote has been made possible after officials in the Council of Ministers were able to speed up work on the EU's data protection legislation.
The proposed directive would oblige airlines to hand EU countries their passengers' data in order to help the authorities to fight terrorism and serious crime.
Under the Commission proposal, air carriers operating flights between a third country and the territory of at least one EU member state would be obliged to send PNR data to the competent authorities of that member state.