Israel wants to maintain a naval blockade on Gaza as it claims Hamas, which won elections in 2006, cannot be trusted to prevent prohibited materials from reaching terror groups. Hamas itself is designated as a terror organisation by the European Union.
The Estelle, a Finnish-flagged vessel carrying supplies and activists towards Gaza, will face opposition from the Israeli military if it continues on its journey towards the Palestinian territory.
"It will not be allowed to go into the Gaza Strip port," said Yuval Donio-Gideon, First Secretary at the Israeli Embassy in Finland.
"Because Gaza is ruled by a terror organization, we cannot make sure what goes into Gaza, we cannot inspect that and cannot trust the authorities in Gaza to inspect that and to make sure that materials would help the terrorists in Gaza would get to their hands."
The blockade of Gaza has been declared illegal by the UN Commissioner for Human Rights as it unfairly penalises the civilian population.
Donio-Gideon declined to give a range within which the ship would be boarded by Israeli forces, citing operational reasons. In 2010 three Turkish citizens were killed after an Israeli raid on a ship making a similar attempt.
Both Israel’s government and the crew of the Estelle view the project as a symbolic more than practical mission. On Tuesday parliamentarians from Norway, Sweden, Spain and Greece joined the ship.