I have found solution with kindly help of Donald Arseneau, author of ulem package. So here it is:
\documentclass[14pt]{extreport}
\usepackage[normalem]{ulem}
\newcommand\s{\nobreak \hskip\fill \allowbreak \space \hskip-\fill \relax}
\begin{document}
\hfill\begin{minipage}[t]{0.7\textwidth}
\begin{flushleft}
\obeyspaces\let =\s
Lorem:~\uline{ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\hfill}
\end{flushleft}
\end{minipage}
\end{document}
and here is result:

but when I try to write environment like this
\newenvironment{under}
{\obeyspaces\let =\s}
{}
\begin{document}
\hfill\begin{minipage}[t]{0.7\textwidth}
\begin{flushleft}
\begin{under}
Lorem:~\uline{ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\hfill}
\end{under}
\end{flushleft}
\end{minipage}
\end{document}
there is an error appearing
! Missing control sequence inserted.
<inserted text>
\inaccessible
l.13 \begin{under}
Why?